Four experimental diets containing calorie/protein ratios of 54, 70, 92 and 142 kcal/kg/% protein were fed to young yellowtail (Seriola quinqueradiata) for 4 weeks. Dietary calorie/protein ratios correlated negatively with weight gain, feed efficiency ratio and apparent lipid retention, and positively with percentage liver lipids. The maximum protein efficiency ratio and its retention were obtained with the experimental diet containing approximately 40% protein and 24% lipid, which is calculated to provide approximately 90 kcal of metabolizable energy for each 1% protein, whereas the maximum energy retention was obtained with the experimental diet containing 55% protein and 17% lipid, which is calculated to provide 70 kcal for each 1% protein. The data indicated that a proper balance between energy and protein was important for optimum growth and feed conversion of yellowtail, as is true for other species, and that it was possible to reduce the protein level of yellowtail diet from 70 to 55% without retardation of growth if the calorie content was maintained with pollack liver oil at a level isocaloric with the original diet.