A feeding experiment was conducted to determine the optimum ratio of energy to protein in diet (DE (kcal)/protein (%)) by feeding rainbow trout on diets with various levels of lipids (5-25%) at a fixed protein level of 35%. The growth rate and feed conversion improved as dietary lipid levels increased. reaching a maximum when 18% lipid was added, indicating that the optimum contents of dietary protein and lipid for rainbow trout are approximately 35% and 18%, respectively. With an increase in the amount of dietary lipids, both the values for PER and NPU increased, giving a maximum protein retention in fish fed a diet containing approximately 18% lipid with the best weight gain. The rate of energy retention in the fish body was around 60%, regardless of the lipid levels in the diets or the growth of the fish. The digestibilities of protein and lipid were as high as 98%, regardless of the lipid contents in the diets. The digestibility of energy increased with an increase in the dietary lipid levels. The experimental value of the optimum DE/protein for the optimum growth of rainbow trout was 130 when based on the measured digestible energy, and the value on the basis of calculated DE, was 100. It has thus been demonstrated that protein can be decreased by approximately 15%, if high-quality lipids, capable of satisfying the EFA requirement of the fish, are added at the level of approximately 18%.