FIRST AMENDMENT TO SENIOR REVOLVING CREDIT AGREEMENT Among ROSETTA RESOURCES INC.,

Exhibit 10.22

Executive Version

F IRST A MENDMENT

TO

S ENIOR R EVOLVING C REDIT A GREEMENT

Among

R OSETTA R ESOURCES I NC .,

as Borrower,

BNP P ARIBAS ,

as Administrative Agent,

and

The Lenders Signatory Hereto

Effective as of September 26, 2005

F IRST A MENDMENT TO S ENIOR R EVOLVING C REDIT A GREEMENT

This F IRST A MENDMENT TO S ENIOR R EVOLVING C REDIT A GREEMENT (this “ First Amendment ”) executed effective as of the 26th of September, 2005 (the “ First Amendment Effective Date ”) is among R OSETTA R ESOURCES I NC ., a corporation formed under the laws of the State of Delaware (the “ Borrower ”); each of the undersigned guarantors (the “ Guarantors ”, and together with the Borrower, the “ Obligors ”); each of the Lenders that is a signatory hereto; and BNP PARIBAS, as administrative agent for the Lenders (in such capacity, together with its successors, the “ Administrative Agent ”).

Recitals

A. The Borrower, the Administrative Agent and the Lenders are parties to that certain Senior Revolving Credit Agreement dated as of July 7, 2005 (the “ Credit Agreement ”), pursuant to which the Lenders have made certain credit available to and on behalf of the Borrower.

B. The Borrower has requested and the Administrative Agent 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 which is defined in the Credit Agreement, but which is not defined in this First Amendment, shall have the meaning ascribed such term in the Credit Agreement. Unless otherwise indicated, all section references in this First Amendment refer to the Credit Agreement.

Section 2. Amendments to Credit Agreement .

2.1 Definitions . Section 1.02 is hereby amended by amending or adding the following definitions:

(a) Applicable Margin . The Borrowing Base Utilization Grid within the definition of “Applicable Margin” is hereby amended and restated in its entirety to read as follows:

Borrowing Base Utilization Grid

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(b) Change in Law . The definition of “Change in Law” is hereby amended by deleting the second “)” after “Section 5.01(b)”.

(c) First Amendment Effective Date . The definition of First Amendment Effective Date is hereby added in the appropriate alphabetical order:

“ ‘ First Amendment Effective Date ’ means the date of that certain First Amendment to Senior Revolving Credit Agreement dated as of September 26, 2005, by and among the Borrower, the Administrative Agent and other Lenders thereto.”

(d) Guarantors. The definition of Guarantors is hereby amended and restated in its entirety as follows:

“ ‘ Guarantors ’ means, collectively:

Rosetta Resources Operating LP;

Rosetta Resources Offshore, LLC;

Rosetta Resources Operating GP, LLC

Rosetta Resources Holdings, LLC;

each Subsidiary that guarantees the Indebtedness pursuant to Section 8.14(b); and

any other Person that must guarantee the Indebtedness in order for the Borrower to comply with Section 9.04(b)(ii)(D).”

(e) Intercreditor Agreement . The definition of “Intercreditor Agreement” is hereby amended and restated in its entirety as follows:

“ ‘ Intercreditor Agreement ’ means in respect of the Second Lien Term Loan Agreement, the terms of subordination as attached as Annex II to the Second Lien Term Loan Agreement, as the same may from time to time be amended, modified, supplemented or restated in accordance with the provisions of Section 9.04(b).”

(f) Second Lien Notes . The definition of “Second Lien Notes” is hereby amended by replacing “$100,000,000” with “$75,000,000”.

(g) Separation . The definition of “Separation” is hereby amended by replacing “Senior” with “Second Lien” in subsection (d)(iii).

2.2 Interim Redetermination of the Borrowing Base . Pursuant to Section 2.07(b), the Borrower elects to initiate, and each Lender consents to, an Interim Redetermination pursuant to

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which the Borrowing Base shall be increased to $325,000,000, effective from and including the First Amendment Effective Date to but excluding the next Redetermination Date. Notwithstanding the foregoing, the Borrowing Base may be subject to further adjustments from time to time pursuant to Section 8.13(c) or Section 9.13.

2.3 Amendment to Section 3.04(c)(iii) . The last sentence of Section 3.04(c)(iii) is hereby amended and restated in its entirety as follows:

“The Borrower shall be obligated to make such prepayment and/or deposit of cash collateral on the date it or any Restricted Subsidiary consummates such disposition; provided that all payments required to be made pursuant to this Section 3.04(c)(iii) must be made on or prior to the Termination Date.”

2.4 Amendment to Section 7.14 . Section 7.14 is hereby amended by adding a “)” after “such Capital Lease”.

2.5 Amendment to Section 7.24 . Section 7.24 is hereby amended and restated in its entirety as follows:

“7.24 Specified Senior Indebtedness . The Indebtedness of the Borrower constitutes “Senior Indebtedness” as defined in the Second Lien Term Loan Documents.”

2.6 Amendment to Section 9.02(g) . Section 9.02(g) is hereby amended by replacing “$100,000,000” with “$75,000,000”.

2.7 Amendment to Section 9.04(b) . Section 9.04(b)(iii) is hereby amended and restated in its entirety as follows:

“(iii) designate any Debt (other than obligations of the Borrower and the Restricted Subsidiaries pursuant to the Loan Documents) as “Senior Indebtedness” or give any such other Debt any other similar designation.”

2.8 Amendment to Section 9.13(b) . Section 9.13(b) is hereby amended and restated in its entirety as follows:

“(b) farmouts in the ordinary course of business of undeveloped acreage or undrilled depths and assignments in connection with such farmouts”

2.09 Amendment to Section 9.21 . Section 9.21 is hereby amended by (a) adding the word “unless” after the first occurrence of the term “Gas Sales Contract” and before subsection (i) and (b) replacing $2,500,000 with $5,000,000 in the second sentence.

2.10 Amendment to Section 12.13 . Section 12.13 is hereby amended by deleting the word “Specified” from the section.

2.11 Amendment to Exhibit F-1 . Exhibit F-1 is hereby amended and restated in i