Along 60 days two growth experiments were conducted to determine the amount of soybean (SBM) and cottonseed (CSM) proteins that could be substituted for fish protein in formulated diets for tilapia (O.niloticus) without reducing growth. In the study, juvenile tilapia (average weight, 10g ) were fed on 32% crude protein diets in which dietary protein was supplied in exp.I by either FM, SBM or mixtures of FM & SBM, while in exp.II FM, CSM or mixtures of FM & CSM were the sources of protein in the diets. Dietary protein in the exp. I was provided as:100% fish meal protein; 75% fish meal protein: 25% SBM protein, 50% fish meal protein: 50% SBM protein; 25% fish meal protein: 75% SBM protein; 100% SBM protein and 100% SBM protein with methionine and lysine supplementation to match the level in the control diet, while in the exp.II, the share of the fish or cottonseed meals as source of protein was planned the same as in the exp.I. In the first experiment, there was no significant difference in the weight gain between the diets containing 25% SBM (26.60g), 50% SBM protein (23.67g) and control diet (26.95g) and the fish performance was better than on diets containing 75% and 100% SBM proteins. Fat content of the body was significantly (P<0.05) decreased with increasing level of SBM protein, while protein content affected only with the level of 100% SBM protein (with or without amino acids suppl.). Protein efficiency ratio of control group (1.78) and fish fed on 25% SBM protein (1.75) was significantly (P<0.05) higher than other treated groups. Apparent protein digestibility was not different among groups fed on FM only or a mixture of FM & SBM proteins. In the second experiment, the best weight gain and feed conversion were obtained with the control group (24.8g, 1.72) fed on FM protein in comparison with the other treated groups fed either on a mixture of FM and CSM or CSM alone. Protein content of the control group was significantly (P<0.05) higher than the other treated groups. Protein efficiency ratio was negatively associated with the increased levels of CSM protein in the diets. Apparent protein digestibility was significantly (P<0.05) decreased as the level of CSM protein increased. It could be concluded that, soybean meal protein could replace up to 50% of the fish meal protein without adverse effect on the growth performance and feed utilization, while cottonseed meal appears to be inefficiently utilized.