A 120-day research was conducted to evaluate the effects of different stocking densities on growth, body composition, survival, yield and economic returns of monosex male Nile tilapia, Oreochromis niloticus in net cages in Kaptai Lake of Bangladesh. Juvenile monosex tilapia with an average weight of 15.20 ± 0.15 g (mean ± SD) were randomly stocked in 12 floating net cages (3 m × 3 m × 2 m) at densities of 50 fish/m 3 (T 50), 75 fish/m 3 (T 75), 100 fish/m 3 (T 100) and 125 fish/m 3 (T 125) in triplicate groups. Fish were fed with a commercial pelleted floating feed (29% protein) at 3-5% of body weight, twice daily in all the treatments. The physico-chemical parameters of lake water were within suitable ranges for fish cultured in cages. After 120 days of trial, growth in terms of body final length, final weight, weight gain, percent weight gain, daily weight gain and specific growth rate of fish from T 50 were significantly higher than those of fish from T 75 , T 100 and T 125. Feed conversion ratio was significantly lower in T 50 followed by T 75 , T 100 and T 125 consecutively. Survival rate was not significantly different in T 50 , T 75 and T 100 while lowest survival was found in T 125. Significantly lower amount of body lipid and carbohydrate contents were found in T 125 than those of fish from T 50 , T 75 and T 100. Gross and net production levels from T 100 were significantly higher than those from T 50 , T 75 and T 125. However, the benefit cost ratio from T 50 was better than those from T 75 , T 100 and T 125. The results demonstrated that on the basis of growth and economic return 50 fish/m 3 was the best stocking density for monosex tilapia culture in cages which might be technically feasible and economically viable.