Exhibit 32.2

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 Mannatech, Incorporated (the “Company”) on Form 10-Q for the period ending June 30, 2008 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Stephen D. Fenstermacher, Chief Financial Officer and Senior Vice President of the Company, hereby certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that:

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 results of operations of the Company.

Date: August 11, 2008

/s/ Stephen D. Fenstermacher

Stephen D. Fenstermacher

Chief Financial Officer and Senior Vice President

A SIGNED ORIGINAL OF THIS WRITTEN STATEMENT REQUIRED BY SECTION 906 HAS BEEN PROVIDED TO

MANNATECH, INCORPORATED AND FURNISHED TO THE SECURITIES AND EXCHANGE COMMISSION OR ITS STAFF

UPON REQUEST.