American Beacon Stephens Small Cap Growth Fund

American Beacon Stephens Mid-Cap Growth Fund

Supplement dated August 22, 2012

to the

Prospectus dated February 24, 2012

The information below supplements the Prospectus dated February 24, 2012 and is in addition to any other supplement(s):

American Beacon Stephens Small Cap Growth Fund

In the “Principal Investment Strategies” section of the Prospectus, the first paragraph is deleted and replaced with the following:

Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowings for investment purposes) in equity securities of small capitalization companies. The Fund considers a company to be a small-cap company if it has a market capitalization, at the time of investment, of $3 billion or less.

American Beacon Stephens Mid-Cap Growth Fund

In the “Principal Investment Strategies” section of the Prospectus, the first paragraph is deleted and replaced with the following:

Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowings for investment purposes) in equity securities of medium capitalization companies. The Fund considers a company to be a mid-cap company if it has a market capitalization, at the time of investment, between $1 billion and the market capitalization of the largest company in the Russell Midcap® Index, which was $20.4 billion as of December 30, 2011.

In the “Principal Investment Strategies” section of the Prospectus, the first sentence in the second paragraph is deleted and replaced with the following:

Most of the assets of the Fund will be invested in U.S. common stocks Stephens Investment Management Group, LLC (“SIMG”) believes have clear indicators of future earnings growth, or that demonstrate other potential for growth of capital.

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PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE