A 3 · 3 factorial experiment was conducted to determine proper levels of dietary protein, lipid and dextrin for juvenile flounder. Nine experimental diets were formulated to contain three protein levels (410, 460 and 510 g kg )1 ) and three lipid levels (60, 130 and 190 g kg )1 ) with corresponding dextrin levels (250, 150 and 50 g kg )1 ). Triplicate groups of fish (8.9 ± 0.4 g) were hand-fed the diets to apparent satiation for 7 weeks in flow-through system. Specific growth rate was the highest in fish fed the 510 g kg )1 protein diet with 60 g kg )1 lipid, and was not significantly different from that of fish fed 460 g kg )1 protein diet with 60 g kg )1 lipid. Feed efficiency ratio tended to increase as dietary protein level increased. The feed efficiency ratio of fish fed the 510 g kg )1 protein diets with 60-190 g kg )1 lipid levels was not significantly different from that of fish fed 460 g kg )1 protein diet with 60 g kg )1 lipid. Daily feed intake tended to decrease with increasing dietary lipid level at each protein level. Daily protein intake increased with increasing dietary protein level at 60 g kg )1 lipid level. Hepatosomatic index and visceralsomatic index increased with increasing dietary lipid level at each protein level. The lipid contents of liver, viscera and whole body, and concentrations of plasma total cholesterol and triglyceride increased with increasing dietary lipid levels; however, no significant difference was observed in the contents of dorsal muscle lipid. The results of this study suggest that the diet containing 460-510 g kg )1 protein with low lipid level (60 g kg )1 ) is optimal for growth and efficient feed utilization of juvenile flounder. KEY WORDS

A 3 (protein levels, 380, 460 and 520 g kg–1 diet) × 2 (lipid levels, 65 and 140 g kg–1 diet) factorial experiment with three replicates was conducted. Weight gain, feed efficiency and daily feed intake were not significantly affected by dietary protein level, but were by dietary lipid level. Weight gains of fish fed 65 g lipid kg–1 diet were significantly, or slightly, higher than for 140 g lipid kg–1 diet at all protein levels. Daily protein intake was significantly affected by both dietary protein and lipid levels (P < 0.002). Daily lipid intake was not significantly affected by dietary protein level, but was by dietary lipid level (P < 0.001). Protein efficiency ratio was significantly affected by dietary protein level (P < 0.02), but not by dietary lipid level. Protein efficiency ratio tended to improve with the decrease of dietary protein level at the same lipid level. Moisture, protein and lipid contents of whole fish were significantly affected by dietary lipid level (P < 0.01). Increased dietary lipid did not improve growth or feed efficiency, but increased body fat deposition. It was concluded that the optimum dietary protein and lipid level for growth of juvenile ayu may be 380 and 65 g kg–1 diet, respectively, when fish were fed to satiety three times daily in seawater.