Abstract: This study assessed the effect of phytase on the growth and enzyme activities of Nile tilapia (Oroechromis niloticus) and on environmental Phosphorus (P) loadings. A basal diet of 30% crude protein, 2.0 g kg ¹ phytate P and total P of 5.6 g kg ¹ was supplemented with phytase (Ronozyme P (5000) CT) at 0, 2000, 4000, 6000 and 8000 units per kg diet. The diets were tested on Nile tilapia (14.0±0.60 g) for 70 days. Results indicated that all fish fed diets with phytase had higher Mean Weight Gain (MWG), Specific Growth Rate (SGR), mineral contents and better feed conversion ratio than the fish fed diet without phytase. The MWG (11.6 g) and SGR (0.86%) of the fish fed diet with 8000 units phytase kg ¹ diet were higher (p< 0.05) than those of the fish fed diets 1 (8.30 g; 0.67%), 2 (8.70 g; 0.69%), 3 (8.90 g; 0.70% and 4 (9.60 g; 0.74%). Analyses of the gut of the fish after the feeding indicated increased amylase, lipase and protease activities in the fish fed diets with phytase in comparison with the fish fed diet without phytase. These activities increased with increase in the level of the phytase. Magnesium (Mg) contents of the fish fed diets with 6000 (72.4 mg g ¹ DM) and 8000 (71.6 mg g ¹ DM) units phytase kg ¹ diet were statistically the same. These contents were however higher (p< 0.05) than those of the fish fed diets 1 (45.2 mg g ¹ DM), 2 (59.6 mg g ¹ DM) and 3 (68.5 mg g ¹ DM). Ca and Zn deposition were the same in all the fish, while Mn contents of the fish fed diets with phytase, diet 2 (8.49 ug g ¹ DM), diet 3 (8.29 ug g ¹ DM), diet 4 (8.80 ug g ¹ DM) and diet 5 (9.35 ug g ¹ DM) were higher (p< 0.05) than that of the fish fed diet 1 (5.50 ug g ¹ DM) without phytase. The Phosphorus (P) load decreased linearly with increase in the level of phytase, such that the P released by fish fed diets 2 (19.5 mg g ¹ DM), 3 (19.1 mg g ¹ DM), 4 (18.9 mg g ¹ DM) and 5 (16.8 mg g ¹ DM) was significantly lower than 24.0 mg g ¹ (DM) P released by fish fed diet 1 without phytase.