EXHIBIT 99.1

Certification of

Chief Executive Officer

of Mannatech, Incorporated

This certification is provided pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, 18 U.S.C. §1350, and accompanies the quarterly report on Form 10-Q (the “Form 10-Q”) for the quarter ended June 30, 2002 of Mannatech, Incorporated (the “Issuer”).

I, Robert M. Henry, the Chief Executive Officer of the Issuer certify that:

(i) the Form 10-Q fully complies with the requirements of section 13(a) or section 15(d) of the Securities Exchange Act of 1934 (15 U.S.C. 78m(a) or 78o(d)); and

(ii) the information contained in the Form 10-Q fairly presents, in all material respects, the financial condition and results of operations of the Issuer.

Dated:  August 14, 2002.

/s/    ROBERT M. HENRY

Robert M. Henry

Subscribed and sworn to before me

this 14 day of August, 2002

[NOTARY SEAL]

/s/    CAROLYN D. STUART

Carolyn D. Stuart

Notary Public

My commission expires:  10/19/02