Abstract: Effects of freshwater and seawater on growth, total testosterone levels, testis development of Oreochromis niloticus, Oreochromis aureus and Tilapia zilli were determined in this study. Although seawater affected the growth of fishes positively (p<0.05) and all testis development stages (immature, maturing and maturated) were observed in all species in seawater, total testosterone concentrations of the fishes grown in this environment did not reach the levels seen in the fishes grown in freshwater (p<0.05). In addition, in seawater, gonad development was observed later and the last 2 gonadal stages were seen together in all species.