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Elimination of uncontrolled reproduction is desirable to channel the available energy for the efficient and rapid growth of tilapias. This may be achieved by hormonal treatment and chromosomal manipulation. For Oreochromis mossambicus, experiments were designed to evaluate the interactions between feeding rate with a diet containing 17a-methyltestosterone (MT), dose and treatment duration. Complete masculinization of firstfeeding fry was obtained by a feeding regime which ensured the uptake of MT of 1.5 µg/g fish/day lasting for 11 days, commencing on the 10th day after hatching. Heat shocking of 2.5-minute old fertilized eggs (42 C for 3 minutes) resulted in the formation of triploids. A technique for ensuring 100% triploidy is described.