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¹ Jeju National University, Department of Marine Life Sciences, Jeju, South Korea DOI : 10.4194/1303-2712-v14_4_02 Viewed : 6422 - Downloaded : 5332 A 9-week feeding trial was conducted in low temperature season to evaluate the effects of varying levels of dietary isoleucine on hematology and innate immune response of juvenile olive flounder. Six isonitrogenous (45% crude protein) and isocaloric (4.45 kcal g^-1 gross energy) diets containing graded levels of isoleucine (0.48, 0.87, 1.43, 1.94, 2.37 and 2.78% dry diet) were formulated. Triplicate groups of fish (8.59 ± 0.13 g) were fed the six test diets to apparent satiation twice daily. Fish growth performance was affected by dietary isoleucine levels and the highest growth was obtained at 1.43% isoleucine. The results showed the significant (P<0.05) increase of plasma total protein and cholesterol concentrations at isoleucine levels of 1.43 - 2.37% compared to those offered 0.48% isoleucine. Also, significant reductions in plasma alanine aminotransferase and aspartate aminotransferase activities were detected by increment of dietary isoleucine up to 1.94%. Fish innate immunity was significantly affected by isoleucine levels. Significantly higher lysozyme activity was found at 0.87 - 2.37% isoleucine and superoxide dismutase activity increased at 1.94-2.37% isoleucine levels. Respiratory burst activity and total immunoglobulin level were significantly enhanced at isoleucine levels of 1.43-1.94 and 1.94%, respectively. The findings in this study showed that inclusion of approximately 2% isoleucine in diets for olive flounder can enhance innate immunity. To estimate isoleucine requirement of the species further studies are required. Keywords : Dietary isoleucine, olive flounder, Paralichthys olivaceus, growth, hematology, innate immunity