To evaluate the effects of dietary available phosphorus(AP)levels on growth performance,body composition and biochemical indices of GIFT tilapia(Oreochromis niloticus),seven practical diets(isonitrogenous and isocaloric)were formulated to contain 0.52%(basal diet,control),0.61%,0.70%,0.78%,0.87%,0.96% and 1.05% of AP supplied with monocalcium phosphate(MCP).Each diet was fed to triplicate groups of 25 fish with an initial average body weight of(29.40±0.15)g for 56 d.The results showed that dietary AP levels significantly affected weight gain rate(WGR),specific growth rate(SGR),feed conversion ratio(FCR)and viscerosomatic index(VSI)(P<0.05).Quadratic regression analysis projected the optimal dietary AP level of tilapia to 0.80% based on WGR.With the increase of dietary AP level,the total phosphorus and crude ash contents of whole body markedly increased and then leveled off(P<0.05),while moisture contents of whole body were not significantly affected(P>0.05),whereas crude lipid contents of the whole body,mesentery and liver significantly decreased(P<0.05).Broken-line analysis showed that dietary AP requirement of tilapia were 0.81% and 0.80% respectively,based on the total phosphorus and crude ash contents of whole body.With increasing dietary AP level,hepatic activities of malate dehydrogenase(MDH),pyruvate kinase(PK),lipoprotein lipase(LPL)and hepatic lipase(HL)markedly increased(P<0.05).With an increment of dietary AP level,activities of serum glutamic-pyruvic transaminase(GPT)significantly decreased(P<0.05),but alkaline phosphatase(AKP)markedly increased(P<0.05).Serum superoxide dismutase(SOD)activity initially increased and then decreased,but liver MDA concentration showed contrary trends to that of SOD as dietary AP level increased.Highest hepatic SOD activity and the lowest liver MDA concentration were both recorded at 0.78% AP.In conclusion,the overall results in this study suggest that the optimal dietary AP level for GIFT tilapia(30-150 g)was 0.80%.