Jamaica red tilapia fry (Oreochromis niloticus x O. mossambicus) x (O. aureus x O. hornorum) of mean length 10.4 mm ± 0.3 SD and weight 0.031 g ± 0.006 SD were fed high protein diets containing 60 mg and 70 mg of 17-? methyltestosterone kg-1 food, respectively, over a 28-day period at water temperatures of 26°C, 30° C, and 34°C. The fish were grown for a further 30 days and dissected to determine their sex by the gonadal squash method. The results of the experiment were analyzed to determine the optimal hormone concentration and water temperature for masculinization, and whether there were any interactions between hormone concentration and water temperature. Growth, survivorship, and masculinization were generally better at the lower hormone concentration. There was some evidence, however, of an interaction between hormone concentration and water temperature, but this interaction was significant only on survivorship. The best survivorship (87%) and masculinization rate (94%) occurred at the temperature of 34°C and hormone concentration of 60 mg 17-? methyltesterone kg-1 diet, but the best growth occurred at 30°C at the lower hormone concentration.