An 8-week feeding trial was conducted to estimate the optimum dietary protein to energy (P/E) ratio in juvenile olive £ounder Paralichthys olivaceus. Eight experimental diets were formulated with two energy levels and four protein levels at each energy level. Two energy levels of 12.5 and 16.7 kJ g À 1 diets were included at crude protein (CP) levels of 25%, 30%, 35% and 45% with 12.5 kJ g À 1 , and CP levels of 35%, 45%, 50% and 60% with 16.7 kJ g À 1 . After 1 week of the conditioning period, ¢sh initially averaging 8.1 AE 0.08 g (mean AE SD) were randomly distributed into the aquarium as groups of 15 ¢sh. Each diet was fed on a dry-matter basis to ¢sh in three randomly selected aquariums at a rate of 3^5% of total wet body weight per day for 8 weeks.After 8 weeks of the feeding trial, weight gain (WG), feed e⁄ciency ratio and speci¢c growth rate of ¢sh fed 45% CP with16.7 kJ g À 1 energy diet were sig-ni¢cantly higher than those from the other dietary treatments (Po0.05). WG of ¢sh fed 12.5 kJ g À 1 energy diets increased with the increase of dietary protein levels. However,WG of ¢sh fed 16.7 kJ g À 1 energy diets increased with the increase of dietary protein levels up to 45% CP and then decreased when ¢sh fed 50% and 60% CP diets. Both dietary protein and energy a¡ected protein retention e⁄ciency and energy retention e⁄ciency. Haemoglobin (Hb) of ¢sh fed 35% and 45% CP diets with 12.5 kJ g À 1 energy were signi¢cantly high and not di¡erent from Hb of ¢sh fed 45% and 50% CP diets with 16.7 kJ g À 1 energy. Haematocrit of ¢sh fed 45% CP diet with 16.7 kJ g À 1 energy was signi¢cantly higher than those from ¢sh fed 25% and 30% CP diets with 12.5 kJ g À 1 energy (Po 0.05). Based on the results of this experiment, we concluded that the optimum dietary P/E ratio was 27.5 mg protein kJ À 1 with diet containing 45% CP and16.7 kJ g À 1 energy in juvenile olive £ounder. Aquaculture Research, 2004, 35, 250^255 250 r 2004 Blackwell Publishing Ltd ZnSO 4 Á 7H 2 O, 21.9; Fe-citrate, 29.6; Ca-lactate, 303.89; AlCl 3 Á 6H 2 O, 0.15; KIO 3 , 0.15; Na 2 Se 2 O 3 , 0.01; CuCl 2 , 0.2; MnSO 4 Á H 2 O, 2.0; CoCl 2 Á 6H 2 O, 1.0. Aquaculture Research, 2004, 35, 250^255 Optimum dietary P/E ratio in £ounder K-W Kim et al.