Abstract: The research investigated the effect of phytase in roasted groundnut meal diet on nutrient digestibility, serum parameters and bone mineralization. A total of 1612 Clarias gariepinus of average weight 378.56 g were stocked at 26 fish per tank and fed the phytase treated diets at 1.5% body weight. Each diet was fed to duplicate group of fish for 84 days. Significant effect of phytase (ANOVA, p<0.05) and interaction with groundnut meal was observed for all growth parameters (factorial, p<0.05). Increasing fish substitution by groundnut showed significant decline in growth performance (Duncan, p<0.05) with significant decline in weight gain of fish (r1 = 0.436). However, regardless of fish meal substitution, dietary phytase supplementation improved growth of fish with significantly higher weight gain, daily weight gain, specific growth rate and protein efficiency compared to diets with no phytase (Tukey, p<0.05). Growth assessment showed that there was significant increase in weight gain, daily weight gain and feed intake with phytase addition up to 500 FTU g–1 in 10, 20, 30, 40 and 50% groundnut (Duncan, p<0.05) with the lowest growth at 1000 FTU g–1 phytase. Fish fed 60% groundnut meal (roasted) with phytase at 250 FTU g–1 (G6P1), 750 FTU g–1 (G6P3) and 1000 FTU g–1 (G6P4) had better growth performance (Duncan, p<0.05) compared to basal control (G6P0). increasing fish meal substitution with groundnut meal up to 50% improved survival rate compared to fish meal diet, regardless of phytase level (Tukey, p<0.05). Phytase addition to diet increased phosphorus digestibility (Tukey, p>0.05), which correlated positively with phytase level (r = 0.144, p>0.05). There was significant increase in serum phosphorus in all diets with phytase addition (Duncan, p<0.05) and was significantly improved by phytase (r = 0.418, p<0.01) and analyzed phytase activity (r = 0.469, p<0.01). However, significant reduction in bone mineralization with phytase to diets was observed with reduced bone phosphorus and magnesium (Tukey, p<0.05).