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A growth trial was conducted to estimate the optimum requirement of dietary available phosphorus (P) for black seabream (Sparus macrocephalus) in indoor net cages (1.5 m × 1.0 m × 1.0 m).Triplicate groups of black seabream (11.45 g ± 0.02 g) were fed diets containing graded levels (0.18%, 0.36%, 0.54%, 0.72%, 0.89% and 1.07%) of available P to satiation for 8 weeks.The basal diet (diet 1), containing 0.18 % available P, was supplemented with graded levels of monosodium phosphate (NaH2PO4 · 2H2O) to formulate 5 experimental diets.The fish were fed twice daily (08:00 and 16:00) and reared in seawater (salinity, 26 ~ 29 g · L-1) at a temperature of 28 ℃ ± 1 ℃.Dissolved oxygen during the experiment was above 5 mg · L-1 The specific growth rate (SGR), weight gain (WG), feed efficiency (FE) and protein efficiency ratio (PER) were all significantly improved by dietary phosphorus up to 0.54% (P <0.05) and then leveled off beyond this level.Hepatosomatic index (HSI) was inversely correlated with dietary phosphorus levels (P < 0.05).Efficiency of P utilization stabled in fish fed diets containing 0.18% ~0.54% available P and then decreased dramatically with further supplementation of dietary phosphorus.Body composition analysis showed that the whole-body lipid, ash,calcium and phosphorus contents were all significantly affected by dietary available P concentration (P < 0.05), however, no significance was found in whole-body calcium/phosphorus (Ca/P) ratios among all the treatments (P >0.05).Dietary phosphorus levels also affected the mineralization of vertebrae, skin and scale (P < 0.05).Ca/P ratios in vertebrae and scale were not influenced by dietary P supplementation, while skin Ca/P ratio increased statistically with dietary available P levels (quadratic effect, P < 0.001).The blood chemistry analysis showed that dietary available P had distinct effects on enzyme activities of alkaline phosphatase (ALP) and plasma lysozyme (LSZ), as well as contents of triacyglycerol (TG) and total cholesterol (T-CHO) (P < 0.05).Broken-line analysis showed maximum weight gain (WG) was obtained at dietary available P concentrations of 0.55%.Quadratic analysis based on P contents in whole fish,vertebrae or scale indicated that the requirements were 0.81%, 0.87% and 0.88%, respectively.Signs of phosphorus deficiency were characterized by poor growth, slightly reduced mineralization and an increase in body lipid content.