FIRST AMENDMENT TO 5-YEAR REVOLVING CREDIT AGREEMENT

EXHIBIT 10.1

FIRST AMENDMENT TO

5-YEAR REVOLVING CREDIT AGREEMENT

dated as of

March 10, 2006

among

XTO ENERGY INC.,

as Borrower,

JPMORGAN CHASE BANK, N.A.,

as Administrative Agent,

and

The Lenders Party Hereto

BANK OF AMERICA, N.A.,

BNP PARIBAS,

CALYON NEW YORK BRANCH,

and

WACHOVIA BANK, NATIONAL ASSOCIATION,

as Co-Syndication Agents

and

CITIBANK, N.A.,

HARRIS NESBITT FINANCING, INC.,

and

SUNTRUST BANK,

as Co-Documentation Agents

J.P. MORGAN SECURITIES INC. and BANC OF AMERICA SECURITIES LLC,

as Co-Arrangers and Joint Bookrunners

FIRST AMENDMENT TO 5-YEAR REVOLVING CREDIT AGREEMENT

THIS FIRST AMENDMENT TO 5-YEAR REVOLVING CREDIT AGREEMENT (this “ First Amendment ”) dated as of March 10, 2006, is among XTO ENERGY INC. , a Delaware corporation, as the Borrower; JPM ORGAN C HASE B ANK , N.A. , as Administrative Agent, JPM ORGAN C HASE B ANK , N.A. , B ANK OF A MERICA , N.A., and U.S. B ANK N ATIONAL A SSOCIATION , as Issuing Banks, and the Lenders party hereto.

R E C I T A L S

A. The Borrower, the Administrative Agent and the Lenders are parties to that certain 5-Year Revolving Credit Agreement dated as of April 1, 2005 (the “ Credit Agreement ”), pursuant to which the Lenders have made certain loans to and extensions of credit for the account of the Borrower.

B. The Borrower has requested and the Lenders have agreed to amend certain provisions of the Credit Agreement.

C. NOW, THEREFORE, in consideration of the premises and the mutual covenants herein contained, for good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows:

Section 1. Defined Terms . Each capitalized term used herein but not otherwise defined herein has the meaning given such term in the Credit Agreement. Unless otherwise indicated, all article and section references in this First Amendment refer to articles and sections of the Credit Agreement.

Section 2. Amendments to Credit Agreement .

2.1 Amendments to Section 1.01 .

(a) The definition of “ Agreement ” is hereby amended in its entirety to read as follows:

“ Agreement ” means this 5-Year Revolving Credit Agreement, as amended by the First Amendment, as the same may from time to time be amended, modified, restated, or replaced from time to time.

(b) The first paragraph of the definition of “ Applicable Margin ” is hereby amended to read as follows:

“ Applicable Margin ” means, for any day, with respect to any Eurodollar Loan, or with respect to the commitment fees payable hereunder, as the case may be, the Applicable Margin per annum set forth below under the caption “Commitment Fee Rate” or “Eurodollar Spread”, as the case may be, based upon the rating by S&P and Moody’s, respectively, applicable on such date to the Index Debt:

provided that for each day during which the Borrower’s utilization of available commitments is greater than or equal to 50%, the then applicable Eurodollar Spread will be increased by 0.05% if the Borrower’s Index Debt Rating is in Category 1, 2 or 3 and will be increased by 0.10% if the Borrower’s Index Debt Rating is in Category 4 or 5.

(c) The definition of “ Indebtedness ” is hereby amended to insert the word “and” prior to clause (l) and to delete the phrase “, and (m) obligations in respect of “ship-or-pay” or “take-or-pay” contracts” in its entirety.

(d) The definition of “ Maturity Date ” is hereby amended in its entirety to read as follows:

“ Maturity Date ” means the later of (a) April 1, 2011 and (b) if maturity is extended pursuant to Section 2.05, such extended maturity date as determined pursuant to Section 2.05 (it being understood and agreed that any such maturity shall not be deemed extended for any Lender that has not consented to such extension).

(e) The definitions of “ Investments ” and “ Permitted Investments ” are hereby deleted.

(f) The following definition is hereby added where alphabetically appropriate to read as follows:

“ First Amendment ” means the First Amendment to 5-Year Revolving Credit Agreement dated as of March 10, 2006 among the Borrower and the Lenders party thereto.

2

2.2 Amendment to Section 1.04. Section 1.04 is hereby deleted and the following inserted in lieu thereof:

Section 1.04 Accounting Terms; GAAP . Except as otherwise expressly provided herein, all terms of an accounting or financial nature shall be construed in accordance with GAAP, as in effect from time to time; provided that, if the Borrower notifies the Administrative Agent that the Borrower requests an amendment to any provision hereof to eliminate the effect of any change occurring after the date hereof in GAAP (including but not limited to any Statement of Financial Accounting Standards) or i