A feeding trial of three dietary protein levels (30, 40 and 50%) and two dietary energy levels (19 and 21 MJ kg 1 diet) using a factorial design with three replications was conducted to investigate the effect of dietary protein and energy levels for the growth of juvenile masu salmon (Oncorhynchus masou, Brevoort). Fish, average weight 21.9 g, were fed the experimental diets for 10 weeks. The weight gain and feed efficiency ratio of fish improved as dietary protein and energy levels increased (P < 0.05), whereas there was no significant difference between fish fed the diets containing 40 and 50% protein at the same dietary energy level. Weight gain of fish fed the high-energy diets was significantly higher than those fed the low-energy diets at 40 and 50% dietary protein levels. The results of this study indicate that a diet containing 40% protein and 21 MJ kg 1 diet should be optimal for the growth and effective protein utilisation of juvenile masu salmon (Lee and Kim 2001).