Exhibit 99.1 CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002 The undersigned officers of ATP Oil & Gas Corporation (the "Company"), do hereby certify in accordance with 18 U.S.C. Section 1350, as adopted pursuant to 906 of the Sarbanes-Oxley Act of 2002, that the foregoing Annual Report on Form 10-K of the Company for the year ended December 31, 2002: 1) fully complies with the requirements of section 13 (a) or 15 (d) of the Securities Exchange Act of 1934; and 2) the information contained in such Annual Report on Form 10-K for the year ended December 31, 2002, fairly represents, in all material respects, the financial condition and the results of operations of the Company. Date: March 31, 2003 By: /s/ T. Paul Bulmahn -------------------------------------- T. Paul Bulmahn Chairman, Chief Executive Officer and President Date: March 31, 2003 By: /s/ Albert L. Reese, Jr. -------------------------------------- Albert L. Reese, Jr. Senior Vice President and Chief Financial Officer