Abstract: The effect of phytase addition in toasted soybean meal based diets on growth performance of Nile tilapia Oreochromis niloticus fingerlings (6.22?0.04) were investigated for 63 days in indoor glass tanks. Phytase was incorporated at 0, 2000, 40000, 6000,8000 and 10000 units/kg diet making up diets 1-6. The diets were fed to the fish at 5% of their body weight, twice daily. The growth indices such as mean weight gain, specific growth rate and food conversion ratio were used to assess the growth performance of the fishes. Results indicated no significant differences (p>0.05) in the mean weight gain , specific growth rate, and food conversion ratio of all fish fed the various diets. The apparent digestibility coefficient (ADC) for protein of all fish fed diets with phytase were higher than that in the fish fed diet without phytase. Fish fed diets 3, 4, 5 and 6 had significantly higher ADC for protein than the ADC for protein of fish fed diets 1 and 2. Generally, the mineral compositions in the faeces of fish fed diets with phytase were generally lower than that of group of fish fed diet without phytase. However, there was no clear effect of phytase on the mineral deposition in the body of the fish. The results of the study revealed that addition of phytase to feeds marginally improved the growth performance of the Nile tilapia and reduced the mineral compositions in the faeces of the fish.