he study investigated the effect of dietary protein levels on growth performance and body composition of the Nile tilapia, Oreochromis niloticus fingerlings (8.0 g), using a recirculating water system. The system consisted of eight rearing concrete tanks, underground plastic tank acts as a clarifier and upper plastic tank acts as a biofilter. The system was supplied with air and water pumps. Fish were fed on four dietary protein levels (20%, 25%,
30% and 35%) to satiaty for 6 days a week for 6 months. Results showed that fish growth was significantly affected by protein level. Fish fed on 30% protein diet attained the highest growth (higher weight gain, higher daily weight gain
and higher specific growth rate). The best food conversion ratio (FCR) (2.05) was obtained by 30% protein diet, while the poorest FCR (2.72) was obtained by 20% protein diet. Protein efficiency ratio (PER) was significantly affected by
protein levels. It decreased with increasing protein level. Results of body composition showed that protein and lipid contents were significantly affected by protein level. Protein content was significantly increased with increasing
protein level in the diet. In contrast, lipid content was inversely affected by increasing the dietary protein level. The study showed that the recirculating water system is suitable for Nile tilapia culture in Egypt, and that a diet
containing 30% crude protein is economical and suitable for Nile tilapia fingerlings feeding.