ABSTRACT: The optimum dietary protein level for the growth of young Japanese flounder was investigated. Three replicate groups of fish (average weight 22.7 g) were fed six isocaloric diets containing various protein levels from 40% to 65% for 9 weeks. Casein and white fishmeal were used as the dietary protein sources. Weight gain and feed efficiency of fish fed diets with 45% and 50% protein levels were significantly higher than the other groups (P < 0.05). Protein efficiency ratio of fish fed diets with 55%, 60%, and 65% protein levels was significantly lower than that of other groups (P < 0.05). Plasma total protein concentration increased with increasing dietary protein level up to 50% and glucose concentration decreased with increased dietary protein, probably as a result of the dietary protein and carbohydrate contents. Proximate composition of dorsal muscle was not significantly affected by dietary protein level (P > 0.05), but moisture, protein and lipid contents of liver were significantly affected (P < 0.05). The study’s results indicate that a dietary protein level of 45% can be recommended for optimum growth and efficient protein utilization of flounder growing between 22.7 g and 110 g.