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An 8-week trial was conducted to compare the efficacy of a chelated copper (Cu) with the inorganic Cu (CuSO4), in the diet of Japanese sea bass ( Lateolabrax japonicus). Six diets were prepared based on two basal diets (semi-purified or practical) at two Cu inclusion levels (0 and 4 mg kg−1 from or chelated-Cu), accordingly named as S0, SI4, SM4, P0, PI4 and PM4 respectively. Each diet was fed to quadruplicate groups of Japanese sea bass (initial weight: 65.0 ± 0.03 g) during the trial. Fish fed practical diets had higher feed intake, specific growth rate, protein efficiency rate ( PER), liver Cu accumulation, liver Na+K+- ATPase and plasma alkaline phosphatase, but lower hepatosomatic index than the groups fed semi-purified diets ( P < 0.05). Higher PER was exhibited by fish fed chelated-Cu diets than that of fish fed CuSO4 diets ( P < 0.05). Although liver Cu accumulation was significantly higher in fish fed practical diets,, liver Cu content increased significantly with increasing dietary copper levels only in fish fed semipurified diets. Chelated-Cu has higher efficiency than CuSO4 when fed in practical diets. Poor growth performance of fish fed semipurified diets might induce underestimating the nutrient requirement of Japanese sea bass. [ABSTRACT FROM AUTHOR]