The relationships between dietary energy sources and the utilization of dietary proteins have been investigated with rainbow trout and carp. In rainbow trout, lipid was utilized more effectively than carbohydrate (α-starch and dextrin) as an energy source for utilization of dietary protein. The values for PER and NPU of casein at low protein levels were markedly different according to the kind of energy sources used. Accordingly, the protein level producing the maximum growth rate also differed according to the main energy sources used. On the other hand, carp utilized carbohydrate effectively as an energy source.