Tilapia mossambica fry were treated for the first 69 days of life with 0, 10, 20, 30, 40, and 50 •zg of methyl testosterone per gram of food in order to change the sex of presumptive females to functional males. After treatment, the fish were raised to maturity, the males were bred with normal females, and the offspring of 24 pairs were raised to maturity and sexed. Seven males sired all-female offspring. This strongly indicates that sex direction has been achieved and that the female is the homogametic sex.

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