Abstract:Haemocyte count, phenoloxidase activity, respiratory burst (release of superoxide anion), superoxide dismutase (SOD) activity, glutathione peroxidase (GPX) activity, phagocytic activity and clearance efficiency to the pathogen Vibrio alginolyticus were measured in white shrimp Litopenaeus vannamei juveniles (12.3 ± 1.2 g) which had been fed diets containing sodium alginate at 0.5, 1.0, 2.0 g kg−1 after five months. L. vannamei fed a diet containing 2.0 g kg−1 sodium alginate had increased phenoloxidase activity, respiratory burst and SOD activity, but decreased GPX activity significantly. L. vannamei fed a diet containing 2.0 g kg−1 sodium alginate had increased phagocytic activity and the shrimp fed a diet containing sodium alginate at 0.5, 1.0 or 2.0 g kg−1 had increased clearance efficiency to V. alginolyticus. In another experiment, L. vannamei, which had been fed control diet, or sodium alginate-containing diets after 5 months, were challenged with V. alginolyticus at 2 × 106 colony-forming units (CFU) shrimp−1 and then placed in seawater of 15‰. The survival of shrimp fed a diet containing 2.0 g kg−1 after one day, and the survival of shrimp fed diets containing sodium alginate at 0.5 and 1.0 g kg−1 after 2–4 days increased significantly, as compared to that of shrimp fed control diet. It is therefore concluded that administration of sodium alginate in the diet at 2.0 g kg−1 or less could enhance the immune ability of L. vannamei and increase its resistance to V. alginolyticus infection.