Studies were conducted to investigate the effects of dietary vitamin C(V_C)on growth,immunity and anti-ammonia-nitrite stress ability in juvenile black carp,Mylopharyngodon piceus(initial average weight 7.27?0.40 g).Six iso-nitrogenous and iso-energetic diets were formulated to supply at dosages of 0(control),16.3,33.9,69.1,137.8 and 277.5 mg/kg vitamin C diet,respectively.After feeding 8 weeks,according to specific growth rate,three group of 0,69.1 and 277.5 mg/kg of black carp were subjected to an ammonia-nitrite stress(20 mg/L)for 24 h.Results indicated that the effective vitamin C content for maximum growth is about 63.0 mg/kg.The vitamin V_C levels in liver,muscle and serum were in positive correlation with V_C content in diet,and no significant differences were observed among fish fed the diets with equal to or higher than 137.8 mg/kg of vitamin V_C.No significant differences in total superoxide dismutase(T-SOD)and malondialdehyde(MDA)were observed among dietary treatments.But V_Ccould significantly increase the catalase(CAT)and glutathione peroxidase(GSH-px)activity(P<0.05).With the V_C content increased,CAT increased at first and then decreased.Glutathione(GSH)and Na⁺/K⁺-ATPase(NKA)increased when V_C concentration in diets were increased,cortisol(COR)were decreased at first and then increased.After ammonia-nitrite stress,each group of the V_C concentration in liver,T-SOD,CAT,GSH-px activity and GSH content in serum and gill NKA activity decreased,serum V_C level,COR and MDA content increased.The T-SOD,CAT and NKA activity,GSH,MDA and COR content significantly changed before and after ammonia nitrite stress.Liver vitamin C concentration in fishes fed V_C diets significantly decreased after stress,however,there are no significant changes in CAT,NKA activity and GSH,cortisol content compared with control group.T-SOD activity in 69.1 mg/kg vitamin C diet had no significant change before and after stress,but GSH-px activity and MDA content had significantly changed.In 277.5 mg/kg vitamin C group,GSH-px activity and MDA content had no significant change except T-SOD activity and serum V_C level.All the results indicated that the effective vitamin C requirement of juvenile black carp for optimal growth might be 63.0 mg/kg diet.Fish have been stressed when put in ammonia nitrite,and V_C concentration in diet could improve immunity and anti ammonia-nitrtie stress ability of black carp.