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



In connection with the Quarterly Report of Marathon Oil Corporation (the
"Company") on Form 10-Q for the period ending March 31, 2003 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), I,
Clarence P. Cazalot, Jr., President and Chief Executive Officer of the Company,
certify, pursuant to 18 U.S.C. ss. 1350, as adopted pursuant to ss. 906 of the
Sarbanes-Oxley Act of 2002, that to the best of my knowledge:

    (1)  The Report 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 the Report fairly presents, in all
material respects, the financial condition and result of operations of the
Company.



 /s/ Clarence P. Cazalot, Jr.
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Clarence P. Cazalot, Jr.
President & Chief Executive Officer
May 14, 2003