This study was conducted over 12 weeks' period in order to studythelysine requirements ofgrowing mono-sex Nile tilapia (Oreochromisniloticus). There were nine treatments (cont. T1 without lysine, T2=0.1 lysine, T3= 0.2lysine, T4=0.3 lysine, T5=0.4 lysine, T6=0.5 lysine, T7=0.6 lysine, T8=0.7 lysine, and T9=0.8 lysine). Each treatment consisting of three replicates. Initial body weight of fish was 10 g,stocking rate was10fish/aquariumand daily feeding rate was3% of fish live body weight. Fish were fed a balanced diet of 20% protein. The results indicated that the best fish production, chemical composition, and physiological status were obtained byT6 and T7 (level of the added lysine 0.5 and 0.6% of the diet, i.e. 1.74& 1.84 of the diet as total content of lysine).