Several studies have demonstrated that steroid hormones can influence sex differentiation in nonmammalian vertebrates and it has been hypothesized that male and female sex differentiation are driven by androgen and estrogen hormones, respectively. Estrogen biosynthesis is mediated by the steroidogenic enzyme cytochrome P450 aromatase, which converts androgens to estrogens. In the present study we examined the efficacy of a potent nonsteroidal aromatase inhibitor incorporated into the food, on sex reversal of Nile tilapia (Oreochromis niloticus) larvae. Nile tilapia larvae were divided in seven groups, which were fed with diets containing different amounts of the aromatase inhibitor Fadrozole (0, 50, 75 and 100 mg/kg) during 15 and 30 days, starting 9 days after hatching. Independent of the period, the proportion of males was significantly higher in the treated groups. Treatment with the highest doses (75 and 100 mg/kg) for 30 days produced 100% males. Histological examination revealed no differences in gonadal tissues between control males and treated fish. Furthermore, one intersex fish was identified in the group treated with 50 mg Fadrozole/kg for 30 days. This study reports that a 100% Nile tilapia male population can be obtained by suppressing aromatase activity and suggests that besides steroid hormones, nonsteroidal compounds, such as aromatase inhibitors, have potential for production of monosex population in tilapia. J. Exp. Zool. 290:177–181, 2001. © 2001 Wiley-Liss, Inc.