The effects of fresh water and sea water on growth, estradiol-17beta levels, ovarian and oocyte development of Oreochromis niloticus, O. aureus and Tilapia zilli were determined. Sea water affected growth positively (p<0.05) but estradiol-17beta concentrations in fish reared in sea water did not reach the levels in fish reared in fresh water. Ovarian stages developed differently in sea and fresh water. Ovulation occurred only when the fish were grown in fresh water whereas mature and atretic oocytes were seen simultaneously in all species when grown in sea water. There were no statistically significant differences in oocyte diameter between the groups grown in fresh and sea water (p>0.05).