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Oreochromis aureus swim-up fry were fed 0, 1, 10 or 60 ppm 17a-methyltestosterone (MT) for 30 days. Fish
that had been fed 0 ppm MT for the first 30 days (phase I) were subsequently fed 0 ppm MT (0-0), 10 ppm MT
(0-10), or 60 ppm MT (0-60) for the next 57 days (phase II); fish that had been fed 60 ppm MT during phase I
were subsequently fed 0 ppm MT (60-0), 10 ppm MT (60-10), or 60 ppm MT (60-60) during phase II. One group
of fish was fed 1 ppm MT {1-1) and another group was fed 10 ppm MT {10-10) during the entire 87-day
experiment. The 60-0, 60-10 and 60-60 treatments produced monosex male populations. The 10-10 treatments
produced a 99%-male population. The 0-10, 0-60 and 1-1 ppm MT treatments did not alter the sex ratio. During
phase I, average weight increased significantly as MT concentration increased. The group fed 60 ppm MT grew
16.4% more than the nontreated control. Final weights for all MT-treated groups were significantly greater
than that of the nontreated group. Ten ppm MT-treated feed may produce 100% male populations if the
hormone is administered when fry begin to feed and if fish are fed to satiation. In the 0-10 and 0-60 treatment,
females had greater anabolic response than males.