Exhibit 10.1

EXECUTION COPY

REVOLVING CREDIT AGREEMENT

(3 Year Facility)

dated as of October 18, 2005

among

ATMOS ENERGY CORPORATION,

as Borrower

THE LENDERS FROM TIME TO TIME PARTY HERETO,

SUNTRUST BANK,

as Administrative Agent

JPMORGAN CHASE BANK, N.A.,

as Syndication Agent,

AND

BANK OF AMERICA, N.A.,

WACHOVIA BANK, NATIONAL ASSOCIATION

and

SOCIÉTÉ GÉNÉRALE,

as Co-Documentation Agents

SUNTRUST CAPITAL MARKETS, INC.

and

J.P. MORGAN SECURITIES INC.,

As Co-Lead Arrangers and Co-Book Managers

TABLE OF CONTENTS

ii

Schedules

Exhibits

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REVOLVING CREDIT AGREEMENT

THIS REVOLVING CREDIT AGREEMENT (this “ Agreement ”) is made and entered into as of October 18, 2005, by and among ATMOS ENERGY CORPORATION, a Texas and Virginia corporation (the “ Borrower ”), the several banks and other financial institutions and lenders from time to time party hereto (the “ Lenders ”), and SUNTRUST BANK, in its capacity as administrative agent for the Lenders (the “ Administrative Agent ”).

W I T N E S S E T H:

WHEREAS, the Borrower has requested that the Lenders establish a $600,000,000 revolving credit facility in favor of the Borrower;

WHEREAS , subject to the terms and conditions of this Agreement, the Lenders, to the extent of their respective Commitments as defined herein, are willing severally to establish the requested revolving credit facility in favor of the Borrower.

NOW, THEREFORE , in consideration of the premises and the mutual covenants herein contained, the Borrower, the Lenders and the Administrative Agent agree as follows:

ARTICLE I

DEFINITIONS; CONSTRUCTION

Section 1.1. Definitions . In addition to the other terms defined herein, the following terms used herein shall have the meanings herein specified (to be equally applicable to both the singular and plural forms of the terms defined):

“ Additional Commitment Amount ” shall have the meaning given to such term in Section 2.19 .

“ Additional Lender ” shall have the meaning given to such term in Section 2.19 .

“ Adjusted LIBO Rate ” shall mean, with respect to each Interest Period for a Eurodollar Borrowing, the rate per annum obtained by dividing (i) LIBOR for such Interest Period by (ii) a percentage equal to 1.00 minus the Eurodollar Reserve Percentage.

“ Administrative Questionnaire ” shall mean, with respect to each Lender, an administrative questionnaire in the form prepared by the Administrative Agent and submitted to the Administrative Agent duly completed by such Lender.

“ Affiliate ” shall mean, as to any Person, any other Person directly or indirectly controlling, controlled by or under direct or indirect common control with such Person. A Person shall be deemed to control another Person if such Person possesses, directly or indirectly, the power (a) to vote 10% or more of the securities having ordinary voting power for the election

of directors of such other Person or (b) to direct or cause direction of the management and policies of such other Person, whether through the ownership of voting securities, by contract or otherwise.

“ Aggregate Commitment Amount ” shall mean the aggregate principal amount of the Aggregate Commitments from time to time. On the Closing Date, the Aggregate Commitment Amount equals $600,000,000.

“ Aggregate Commitments ” shall mean, collectively, all Commitments of all Lenders at any time outstanding.

“ Applicable Lending Office ” shall mean, for each Lender and for each Type of Loan, the “Lending Office” of such Lender (or an Affiliate of such Lender) designated for such Type of Loan in the Administrative Questionnaire submitted by such Lender or such other office of such Lender (or an Affiliate of such Lender) as such Lender may from time to time specify to the Administrative Agent and the Borrower as the office by which its Loans of such Type are to be made and maintained.

“ Applicable Margin ” shall mean, as of any date, the percentage per annum determined by reference to the applicable Rating Category from time to time in effect as set forth on Schedule I ; provided , that a change in the Applicable Margin resulting from a change in the Rating Category shall be effective on the day on which any rating agency changes its rating and shall continue until the day prior to the day that a further change becomes effective. Notwithstanding the foregoing, the Applicable Margin from the Closing Date until the first change in the applicable Rating Category after the Closing Date shall be at Level III as set forth on Schedule I .

“ Applicable Percentage ” shall mean, as of any date, with respect to the unused commitment fee as of any date, the percentage per annum determined by reference to the applicable Rating Category as set forth on Schedule I ; provided , that a change in the Applicable Percentage resulting from a change in the Rating Category shall be effective on the day on which either rating agency changes its rating and shall continue until the day prior to the day that a further change becomes effective. Notwithstanding the foregoing, the Applicable Percentage for the unused commitment fee from the Closing Date until the first change in the applicable Rating Category after the Closing Date shall be at Level III as set forth on Schedule I .

“ Approved Fund ” shall mean any Person (other than a natural Person) that is (or will be) engaged in making, purchasing, holding or otherwise investing in commercial loans and similar extensions of credit in the ordinary course of its business and that is administered or managed by (i) a Lender, (ii) an Affiliate of a Lender or (iii) an entity or an Affiliate of an entity that administers or manages a Lender.

“ Assignment and Acceptance ” shall mean an assignment and acceptance entered into by a Lender and an assignee (with the consent of any party whose consent is required by Section 9.4(b) ) and accepted by the Administrative Agent, in the form of Exhibit B attached hereto or any other form approved by the Administrative Agent.

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“ Availability Period ” shall mean the period from the Closing Date to the Commitment Termination Date.

“ Bankruptcy Code ” shall mean the Bankruptcy Code in Title 11 of the United States Code, as amended, modified, succeeded or replaced from time to time.

“ Base Rate ” shall mean the higher of (i) the per annum rate which the Administrative Agent publicly announces from time to time to be its prime lending rate, as in effect from time to time, and (ii) the Federal Funds Rate, as in effect from time to time, plus one-half of one percent (0.50%). The Administrative Agent’s prime lending rate is a reference rate and does not necessarily represent the lowest or best rate charged to customers. The Administrative Agent may make commercial loans or other loans at rates of interest at, above or below the Administrative Agent’s prime lending rate. Each change in the Administrative Agent’s prime lending rate shall be effective from and including the date such change is publicly announced as being effective.

“ Borrowing ” shall mean a borrowing consisting of Loans of the same Type, made, converted or continued on the same date and in the case of Eurodollar Loans, as to which a single Interest Period is in effect.

“ Business Day ” shall mean (i) any day other than a Saturday, Sunday or other day on which commercial banks in Atlanta, Georgia and New York, New York are authorized or required by law to close and (ii) if such day relates to a Borrowing of, a payment or prepayment of principal or interest on, a conversion of or into, or an Interest Period for, a Eurodollar Loan or a notice with respect to any of the foregoing, any day on which dealings in Dollars are carried on in the London interbank market.

“ Capital Stock ” shall mean (a) in the case of a corporation, all classes of capital stock of such corporation, (b) in the case of a partnership, partnership interests (whether general or limited), (c) in the case of a limited liability company, membership interests and (d) any other interest or participation that confers on a Person the right to receive a share of the profits and losses of, or distributions of assets of, the issuing Person.

“ Change in Law ” shall mean (i) the adoption of any applicable law, rule or regulation after the date of this Agreement, (ii) any change in any applicable law, rule or regulation, or any change in the interpretation or application thereof, by any Governmental Authority after the date of this Agreement, or (iii) compliance by any Lender (or its Applicable Lending Office) (or for purposes of Section 2.15(b ), by such Lender’s parent corporation, if applicable) with any request, guideline or directive (whether or not having the force of law) of any Governmental Authority made or issued after the date of this Agreement.

“ Change of Control ” shall mean either of the following events:

(a) any “person” or “group” (within the meaning of Section 13(d) or 14(d) of the Exchange Act) has become, directly or indirectly, the “beneficial owner” (as defined in Rules 13d-3 (other than subsection (d) thereof) and 13d-5 under the Exchange Act), by way of merger, consolidation or otherwise of 40% or more of the voting power of the Borrower on a fully-diluted basis, after giving effect to the conversion and exercise

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of all outstanding warrants, options and other securities of the Borrower convertible into or exercisable for voting stock of the Borrower (whether or not such securities are then currently convertible or exercisable); or

(b) during any period of two consecutive calendar years, individuals who at the beginning of such period constituted the board of directors of the Borrower together with any new members of such board of directors whose elections by such board of directors or whose nomination for election by the stockholders of the Borrower was approved by a vote of a majority of the members of such board of directors then still in office who either were directors at the beginning of such period or whose election or nomination for election was previously so approved cease for any reason to constitute a majority of the directors of the Borrower then in office.

“ Charges ” shall have the meaning set forth in Section 9.12 .

“ Closing Date ” shall mean the date on which the conditions precedent set forth in Section 3.1 and Section 3.2 have been satisfied or waived in accordance with Section 9.2 .

“ Code ” shall mean the Internal Revenue Code of 1986, as amended from time to time, and the rules and regulations promulgated thereunder.

“ Commitment ” shall mean, with respect to each Lender, the obligation of such Lender to make Loans to the Borrower in an aggregate principal amount not exceeding the amount set forth with respect to such Lender on Schedule II , as such schedule may be amended pursuant to Section 2.19 , or in the case of a Person becoming a Lender after the Closing Date through an assignment of an existing Commitment, the amount of the assigned “Commitment” as provided in the Assignment and Acceptance executed by such Person as an assignee, as the same may be increased or deceased pursuant to terms hereof.

“ Commitment Termination Date ” shall mean the earliest of (i) October 18, 2008, (ii) the date on which the Commitments are terminated pursuant to Section 2.6 and (iii) the date on which all amounts outstanding under this Agreement have been declared or have automatically become due and payable (whether by acceleration or otherwise).

“ Credit Exposure ” shall mean, with respect to any Lender at any time, the outstanding principal amount of such Lender’s Loans.

“ Compliance Certificate ” shall mean a certificate from a Financial Officer of the Borrower in the form of, and containing the certifications set forth in, the certificate attached hereto as Exhibit 5.1(c) .

“ Consolidated Capitalization ” shall mean, without duplication, the sum of (a) all of the shareholders’ equity or net worth of the Borrower and its Subsidiaries on a consolidated basis, as determined in accordance with GAAP plus (b) the aggregate principal amount of Preferred Securities plus (c) the aggregate Minority Interests in Subsidiaries plus (d) Consolidated Funded Debt.

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“ Consolidated Funded Debt ” shall mean, without duplication, the sum of (a) all indebtedness of the Borrower and its Subsidiaries for borrowed money, (b) all purchase money indebtedness of the Borrower and its Subsidiaries (other than trade accounts payable), (c) the principal portion of all obligations of the Borrower and its Subsidiaries under capital leases, (d) all commercial letters of credit and all performance and standby letters of credit issued or bankers’ acceptances created for the account of the Borrower or one of its Subsidiaries, including, without duplication, all unreimbursed draws thereunder, (e) all Guaranty Obligations of the Borrower and its Subsidiaries with respect to funded indebtedness of another Person of the types listed in clauses (a) through (d), (f) all indebtedness of another entity secured by a Lien on any property of the Borrower or any of its Subsidiaries whether or not such indebtedness has been assumed by the Borrower or any of its Subsidiaries, (g) all indebtedness of any partnership or unincorporated joint venture to the extent the Borrower or one of its Subsidiaries is legally obligated with respect thereto, net of any assets of such partnership or joint venture and in the case of the Capital Stock of such partnership or joint venture being held by a Subsidiary, limited to the net worth of such Subsidiary, (h) all obligations of the Borrower and its Subsidiaries to advance or provide funds or other support for the payment or purchase of funded indebtedness (including, without limitation, maintenance agreements, comfort letters or similar agreements or arrangements) (other than as may be given in respect of Atmos Energy Marketing, LLC (“AEM”)) and (i) the principal balance outstanding under any synthetic lease, tax retention operating lease, off-balance sheet loan or similar off-balance sheet financing product of the Borrower or one of its Material Subsidiaries where such transaction is considered borrowed money indebtedness for tax purposes but is classified as an operating lease in accordance with GAAP; provided that neither the indebtedness of AEM incurred in connection with the purchase of gas by AEM for resale to the Borrower nor the guaranty by the Borrower or one of its Subsidiaries of such indebtedness shall be included in this definition if such indebtedness has been outstanding for less than two months from the date of its incurrence by AEM.

“ Consolidated Net Property ” shall mean the Fixed Assets less, without duplication, the amount of accumulated depreciation and amortization attributable thereto.

“ Contractual Obligation ” of any Person shall mean any provision of any security issued by such Person or of any agreement, instrument or undertaking under which such Person is obligated or by which it or any of the property in which it has an interest is bound.

“ Credit Documents ” shall mean, collectively, this Agreement, the Notes (if any), all Notices of Borrowing, all Notices of Conversion/Continuation, all Compliance Certificates and any and all other instruments, agreements, documents and writings executed in connection with any of the foregoing.

“ Debt to Capitalization Ratio ” shall mean the ratio of (a) Consolidated Funded Debt to (b) Consolidated Capitalization.

“ Default ” shall mean any act, condition or event that, with the giving of notice or the lapse of time or both, would constitute an Event of Default.

“ Default Interest ” shall have the meaning set forth in Section 2.10 ( b ).

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“ Dollar(s) ” and the sign “$” shall mean lawful money of the United States of America.

“ Environmental Laws ” shall mean any current or future legal requirement of any Governmental Authority pertaining to (a) the protection of health, safety, and the indoor or outdoor environment, (b) the conservation, management, or use of natural resources and wildlife, (c) the protection or use of surface water and groundwater or (d) the management, manufacture, possession, presence, use, generation, transportation, treatment, storage, disposal, release, threatened release, abatement, removal, remediation or handling of, or exposure to, any hazardous or toxic substance or material or (e) pollution (including any release to land surface water and groundwater) and includes, without limitation, the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended by the Superfund Amendments and Reauthorization Act of 1986, 42 USC 9601 et seq., Solid Waste Disposal Act, as amended by the Resource Conservation and Recovery Act of 1976 and Hazardous and Solid Waste Amendment of 1984, 42 USC 6901 et seq. , Federal Water Pollution Control Act, as amended by the Clean Water Act of 1977, 33 USC 1251 et seq., Clean Air Act of 1966, as amended, 42 USC 7401 et seq. , Toxic Substances Control Act of 1976, 15 USC 2601 et seq., Hazardous Materials Transportation Act, 49 USC App. 1801 et seq. , Occupational Safety and Health Act of 1970, as amended, 29 USC 651 et seq. , Oil Pollution Act of 1990, 33 USC 2701 et seq. , Emergency Planning and Community Right-to-Know Act of 1986, 42 USC 11001 et seq. , National Environmental Policy Act of 1969, 42 USC 4321 et seq. , Safe Drinking Water Act of 1974, as amended, 42 USC 300(f) et seq. , any analogous implementing or successor law, and any amendment, rule, regulation, order, or directive issued thereunder.

“ ERISA ” shall mean the Employee Retirement Income Security Act of 1974, as amended, and any successor statute thereto, as interpreted by the rules and regulations thereunder, all as the same may be in effect from time to time. References to sections of ERISA shall be construed also to refer to any successor sections.

“ ERISA Affiliate ” shall mean an entity, whether or not incorporated, which is under common control with the Borrower or any of its Subsidiaries within the meaning of Section 4001(a)(14) of ERISA, or is a member of a group which includes the Borrower or any of its Subsidiaries and which is treated as a single employer under Sections 414(b), (c), (m), or (o) of the Code.

“ Eurodollar ” when used in reference to any Loan or Borrowing, refers to whether such Loan, or the Loans comprising such Borrowing, bears interest at a rate determined by reference to the Adjusted LIBO Rate.

“ Eurodollar Reserve Percentage ” shall mean the aggregate of the maximum reserve percentages (including, without limitation, any emergency, supplemental, special or other marginal reserves) expressed as a decimal (rounded upwards to the next 1/100 ^th of 1%) in effect on any day to which the Administrative Agent is subject with respect to the Adjusted LIBO Rate pursuant to regulations issued by the Board of Governors of the Federal Reserve System (or any Governmental Authority succeeding to any of its principal functions) with respect to eurocurrency funding (currently referred to as “eurocurrency liabilities” under Regulation D). Eurodollar Loans shall be deemed to constitute eurocurrency funding and to be subject to such

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reserve requirements without benefit of or credit for proration, exemptions or offsets that may be available from time to time to any Lender under Regulation D. The Eurodollar Reserve Percentage shall be adjusted automatically on and as of the effective date of any change in any reserve percentage.

“ Event of Default ” shall have the meaning provided in Section 7.1 .

“ Exchange Act ” shall mean the Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder.

“ Excluded Taxes ” shall mean with respect to the Administrative Agent, any Lender or any other recipient of any payment to be made by or on account of any obligation of the Borrower hereunder, (a) income or franchise taxes imposed on (or measured by) its net income by the United States of America, or by the jurisdiction under the laws of which such recipient is organized or in which its principal office is located or, in the case of any Lender, in which its applicable lending office is located, (b) any branch profits taxes imposed by the United States of America or any similar tax imposed by any other jurisdiction in which any Lender is located and (c) in the case of a Foreign Lender, any withholding tax that (i) is imposed on amounts payable to such Foreign Lender at the time such Foreign Lender becomes a party to this Agreement, (ii) is imposed on amounts payable to such Foreign Lender at any time that such Foreign Lender designates a new lending office, other than taxes that have accrued prior to the designation of such lending office that are otherwise not Excluded Taxes, and (iii) is attributable to such Foreign Lender’s failure to comply with Section 2.17(e) .

“ Existing Credit Agreement ” shall mean that certain 364-Day Revolving Credit Agreement, dated as of October 22, 2004, among the Borrower, the lenders identified therein and Bank One, NA, as administrative agent, as amended, modified, supplemented or replaced from time to time.

“ Federal Funds Rate ” shall mean, for any day, the rate per annum (rounded upwards, if necessary, to the next 1/100 ^th of 1%) equal to the weighted average of the rates on overnight Federal funds transactions with member banks of the Federal Reserve System arranged by Federal funds brokers, as published by the Federal Reserve Bank of New York on the next succeeding Business Day or if such rate is not so published for any Business Day, the Federal Funds Rate for such day shall be the average rounded upwards, if necessary, to the next 1/100th of 1% of the quotations for such day on such transactions received by the Administrative Agent from three Federal funds brokers of recognized standing selected by the Administrative Agent.

“ Fee Letter ” shall mean that certain fee letter, dated as of September 2, 2005, executed by SunTrust Capital Markets, Inc. and accepted by the Borrower, as modified by the commitment letter, dated as of September 6, 2005, executed by SunTrust Bank, and acknowledged and agreed to by SunTrust Capital Markets, Inc. and the Borrower.

“ Financial Officer ” shall mean any one of the chief financial officer, the controller or the treasurer of the Borrower.

“ Fitch ” shall mean Fitch Ratings Ltd., or any successor or assignee of the business of such company in the business of rating securities.

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“ Fixed Assets ” shall mean the assets of the Borrower and its Subsidiaries constituting “net property, plant and equipment” on the consolidated balance sheet of the Borrower and its Subsidiaries.

“ Foreign Lender ” shall mean any Lender that is not a United States person under Section 7701(a)(3) of the Code.

“ GAAP ” shall mean generally accepted accounting principles in the United States applied on a consistent basis and subject to Section 1.3 .

“ Governmental Authority ” shall mean any Federal, state, local or foreign court or governmental agency, authority, instrumentality or regulatory body.

“ Guaranty Obligations ” shall mean, with respect to any Person, without duplication, any obligations (other than endorsements in the ordinary course of business of negotiable instruments for deposit or collection) guaranteeing any indebtedness for borrowed money of any other Person in any manner, whether direct or indirect, and including without limitation any obligation, whether or not contingent, (a) to purchase any such indebtedness or other obligation or any property constituting security therefor, (b) to lease or purchase property, securities or services primarily for the purpose of assuring the owner of such indebtedness or (c) to otherwise assure or hold harmless the owner of such indebtedness or obligation against loss in respect thereof. The amount of any Guaranty Obligation hereunder shall (subject to any limitations set forth therein) be deemed to be an amount equal to the outstanding principal amount of the indebtedness in respect of which such Guaranty Obligation is made.

“ Hedging Obligations ” shall mean any and all obligations of such Person, whether absolute or contingent and howsoever and whensoever created, arising, evidenced or acquired under (i) any and all Hedging Transactions, (ii) any and all cancellations, buy backs, reversals, terminations or assignments of any Hedging Transactions and (iii) any and all renewals, extensions and modifications of any Hedging Transactions and any and all substitutions for any Hedging Transactions.

“ Hedging Transaction ” shall mean any transaction (including an agreement with respect thereto) now existing or hereafter entered into by such Person that is a rate swap, basis swap, forward rate transaction, commodity swap, interest rate option, foreign exchange transaction, cap transaction, floor transaction, collateral transaction, forward transaction, currency swap transaction, cross-currency rate swap transaction, currency option or any other similar transaction (including any option with respect to any of these transactions) or any combination thereof, whether linked to one or more interest rates, foreign currencies, commodity prices, equity prices or other financial measures.

“ Indemnified Taxes ” shall mean Taxes other than Excluded Taxes.

“ Information Memorandum ” shall mean the Confidential Executive Summary dated September 2005 relating to the Borrower and the transactions contemplated by this Agreement and the other Credit Documents.

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“ Interest Period ” shall mean with respect to any Eurodollar Borrowing, a period of one, two, three or six months; provided, that:

(i) the initial Interest Period for such Borrowing shall commence on the date of such Borrowing (including the date of any conversion from a Borrowing of another Type), and each Interest Period occurring thereafter in respect of such Borrowing shall commence on the day on which the next preceding Interest Period expires;

(ii) if any Interest Period would otherwise end on a day other than a Business Day, such Interest Period shall be extended to the next succeeding Business Day, unless such Business Day falls in another calendar month, in which case such Interest Period would end on the next preceding Business Day;

(iii) any Interest Period which begins on the last Business Day of a calendar month or on a day for which there is no numerically corresponding day in the calendar month at the end of such Interest Period shall end on the last Business Day of such calendar month;

(iv) no Interest Period may extend beyond the Commitment Termination Date.

“ Lenders ” shall have the meaning assigned to such term in the opening paragraph of this Agreement and shall include, where appropriate, each Additional Lender that joins this Agreement pursuant to Section 2.19 .

“ LIBOR ” shall mean, for any applicable Interest Period with respect to any Eurodollar Loan, the British Bankers’ Association Interest Settlement Rate per annum for deposits in Dollars for a period equal to such Interest Period appearing on the display designated as Page 3750 on the Dow Jones Markets Service (or such other page on that service or such other service designated by the British Bankers’ Association for the display of such Association’s Interest Settlement Rates for Dollar deposits) as of 11:00 a.m. (London, England time) on the day that is two Business Days prior to the first day of the Interest Period or if such Page 3750 is unavailable for any reason at such time, the rate which appears on the Reuters Screen ISDA Page as of such date and such time; provided , that if the Administrative Agent determines