Growth performance and feed utilization results are shown in Table 3. Fish fed ≥2.22% arginine exhibited significantly (P b 0.05) higher growth performance than those fed 1.42% arginine. Slight improvements in feed conversion ratio (FCR) and protein efficiency ratio (PER) were obtained by increment of dietary arginine level but the differences were not significant among the treatments. However, fish fed 2.54% arginine showed significantly higher protein productive value (PPV) than fish fed 1.42% arginine. Feed intake and survival did not significantly differ among dietary treatments. The optimal dietary arginine requirement level was estimated to be 2.37% in diet, corresponding to 4.74% of dietary protein, by the broken-line regression analysis on WG (Fig. 1).Whole-body protein content increased in fish fed 2.22–2.54% arginine and differed significantly from that of the fish fed 1.42% arginine (Table 4). A similar trend was observed for muscle protein content, however, the differences were not significant. Whole-body lipid content decreased significantly in groups of fish fed 2.22–2.54% arginine compared to those fed 1.42% arginine and muscle lipid decreased in groups fed 2.22–3.08% arginine. The group of fish fed 1.42% arginine exhibited the lowest muscle moisture content even though the differences were not significant.Plasma total protein level increased significantly at arginine levels of 2.54–3.08% in comparison to fish groups fed ≤1.88 arginine (Table 5). Significantly lower ALT and glucose levels were recorded at dietary arginine levels of 2.22–2.54% and ≥1.88%, respectively. However, hematocrit, hemoglobin, cholesterol and triglyceride levels and AST activity were not significantly influenced by the different levels of dietary arginine.Significantly higher lysozyme and MPO activities were found in groups fed over 2.54% and 2.54–3.08% arginine, respectively, compared to the fish offered 1.42% arginine (Table 6). The groups of fish fed 2.22– 3.08% arginine showed significantly higher plasma Ig levels. Serum T-NOS activity was enhanced by increment of dietary arginine level up to 3.08% and significant differences were found between the groups fed 1.42 and 3.08% arginine (Fig. 2). However, respiratory burst, SOD and antiprotease activities were not significantly influenced by dietary arginine levels.