RESUMO
Nile tilapia [0. niloticus] male production was established by feeding of 17 alpha methyltestosterone at various levels for different durations. Four months from hatching, the fish were sexed by dissection. The fish fed hormonal treated and untreated diets were exposed monthly, from hatching till 120 days of age, to artificial infection with Flexibacter columnaris and Saprolegnia diclin. All male production [100% male] was obtained from the group fed diet containing 30 micro g 17 alpha methyltes-tosterone/g diet for 21 days, while the male percentage in groups fed diets containing 45 and 15 micro g of hormone/g diet for 14 and 42 days were 68 and 56 respectively. The male percent in the control group was 52%. The mortality ratio of artificial infection with F. columnaris and S.diclina in groups fed the hormonal treated and untreated diets were directly proportional with the male percent in each group. Since, the mortalities of F. columnaris and S. diclina infection in the group of all male production at 120 days of age were 40 and 42% respectively, followed by the mortalities in the group of 68% male which were 36 and 43% respectively. The mortality ratio in the control group [52% male] were 30 and 28% for the same organisms respectively and at the same time