The efficacy of androgen hormone in producing monosexed males in Nile tilapia fry and the response of growth and survival rates to hormone treatment were studied. The influence of the androgen treatment on the histological features of the resulting gonads was also evaluated. The synthetic aromatizable 17α-methyltestosterone (MT) was supplemented in diet to newly hatched tilapia fry(<6days) at a level of 500 mg/kg feed for 21 and 56 days treatment periods (G ₁and G₂),while the control group (G₃) was fed untreated diet. About 82% males were produced in groups 1 and 2. Treatment with MT had no significant effect on growth performance. However, MT treatment resulted in lowering the fish survival rate in treated groups (17.5±0.85 and 24.0±0.85 % for G₁and G₂, respectively),than that in the control group(33±0.87%). At the high level of MT used over 21 or 56 days,there were negative structural changes in the gonads.