An all-male population of Tilapia aurea was produced with 17 α-ethynyltestosterone at 60 μg/g diet. Heterogamety of T. aurea females was determined. A relationship between the percentage of males and the total weight of fish indicated the faster growth of androgen-treated fish. The feeding of 1-dehydrotestosterone acetate, 17 α-ethynyltestosterone, and 17 α-methyltestosterone at various dosage levels for 18 days to sexually undifferentiated T. aurea fry in tanks neither affected survival nor altered growth of fry. No difference was observed in survival between androgen-treated and untreated fish in plastic pools for 120 days. Some androgen-treated groups were significantly heavier than the control. Treatment with ethynyltestosterone and methyltestosterone at all dosage levels resulted in significantly higher percentages of males than the control. Degeneration of testes was observed with treatment of methyltestosterone at 60 μg/g diet. No ovotestis condition was found. Sex ratios of the progenies of three ethynyltestosterone-treated and six methyltestosterone-treated males individually bred with untreated females ranged from 2 to 3.1 females:1 male. These were significantly different from the expected 1:1 ratio of sexes and indicated that sex reversal of genetic females had been achieved. Sex reversal appears to be a practical method for producing monosex male tilapia.