EXHIBIT 5.1

[AKIN GUMP LETTERHEAD]

March 26, 2004

Mannatech, Incorporated

600 S. Royal Lane

Suite 200

Coppell, Texas 75019

Ladies and Gentlemen:

We have acted as counsel to Mannatech, Incorporated, a Texas corporation (the “Company”), in connection with the registration, pursuant to a registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”) of up to 1,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share, to be issued under the 1998 Amended and Restated Incentive Stock Option Plan (the “1998 Plan”).

We have examined originals or certified copies of such corporate records of the Company and other certificates and documents of officials of the Company, public officials and others as we have deemed appropriate for purposes of this letter. We have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to authentic original documents of all copies submitted to us as conformed and certified or reproduced copies.

Based upon the foregoing and subject to the assumptions, exceptions, qualifications and limitations set forth hereinafter, we are of the opinion that when the Shares are issued and delivered as described in the 1998 Plan, the Shares will be duly authorized, validly issued, fully paid and non-assessable.

The opinions and other matters in this letter are qualified in their entirety and subject to the following:

A. We express no opinion as to the laws of any jurisdiction other than any published constitutions, treaties, laws, rules or regulations or judicial or administrative decisions (“Laws”) of (i) the Federal Laws of the United States of America or (ii) the Laws of the State of Texas.

B. This law firm is a registered limited liability partnership organized under the laws of the State of Texas.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act and the rules and regulations thereunder.

Sincerely,

AKIN GUMP STRAUSS HAUER & FELD LLP