An 8-week feeding trial was conducted to determine the optimal dietary protein level for juvenile marbled flounder.Five semi-purified test diets were formulated to contain different protein levels (CP)including 42.7%,47.4%,53.3%,58.8%,and 64.5% (dry matter),named as CP42.7,CP47.4,CP53.3,CP58.8,and CP64.5,respectively.Five hundred and twenty-five juveniles (6.0 ± 0.1 g) were randomly distributed into 15 tanks (300 L tanks),resulting in 35 fish per tank (n-3 tanks).Fish were fed the test diets 5 times per day until satiation.The CP58.8 resulted in the highest gain in weight and the best efficiency in feed utilization among the tested protein levels (P < 0.05).Fish fed the CP58.8 diet showed significantly higher whole-body protein and lipid contents than the fish that were fed the other diets (P < 0.05).Fish fed the CP53.3,CP58.8,and CP64.5 diets showed a significantly higher dorsal-muscle lipid content than the fish that were fed the CP42.7 and CP47.4 diets (P < 0.05).The one-slope straight broken-line regression analysis on the results of the thermal growth coefficient and feed conversion ratio indicated that the estimated optimum dietary protein level was 58.8%.Taken together,it is suggested that the dietary protein level of 58.8% is optimal for better growth and high efficiency in feed utilization for the juvenile marbled flounder.