The objectives of the present study were to compare the efficiency of two diets: 1 (NUTRAVIT) and 2 (IP), both with 40% of crude protein, containing the synthetic androgen hormone 17 a-methyltestosterone (MT) and to analyze the most effective dose of this hormone on the sex reversal of Nile tilapia Oreochromis niloticus, throughout histological analysis of the gonads and sex determination of the samples. A total of 9600 Nile tilapia fries with seven days posthatching received the following treatments, per 45 days period: A- 30 mg MT/kg diet 1; B- 60 mg MT/kg diet 1; C- 30 mg MT/kg diet 2; D- 60 mg MT/kg diet 2 and two control groups, E and F with diets 1 and 2 hormone free, respectively. The results of chi-square tests of the frequency data of males and females after the treatments and of analysis of histological and macroscopic characteristics showed that the numbers of males obtained by A, B, C and D treatments was higher than the control groups and the dose of 60 mg MT/kg of diet, as for the diets 1 and 2, was more efficient, resulting in 98% of males. Among diets, the treatment A with diet 1 was better than the treatment C with diet two, for the dose of30 mg MT/kg of diet. There was no difference among diets for the dose of 60 mg MT/kg of the diet.
methyltestosterone; Oreochromis niloticus; sex-reversal; sex-classification