Ameren 10-Q 2003 Q1 Quarterly report

EXHIBIT 99.1

CERTIFICATION PURSUANT TO 18 U.S.C. (S)1350

(ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002)

In connection with the periodic report of Illinois Power Company (the "Company") on Form 10-Q for the period ended March 31, 2003 dated May 15, 2003, filed with the Securities and Exchange Commission (the "Report"), I, Larry F. Altenbaumer, President and Chief Executive Officer of the Company, hereby certify as of the date hereof, solely for purposes of Title 18, Chapter 63, Section 1350 of the United States Code, that to the best of my knowledge:

(1)         the Report fully complies with the requirements of Section 13(a) or 15(d), as applicable, 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 results of operations of the Company at the dates and for the periods indicated.

Date: May 15, 2003	By:	/s/ Larry F. Altenbaumer
		Larry F. Altenbaumer
		President and Chief Executive Officer