The total haemocyte count (THC), differential haemocyte count (DHC), phenoloxidase activity, respiratory burst (release of superoxide anion), superoxide dismutase activity, phagocytic activity and clearance efficiency to the pathogen Vibrio alginolyticus were measured when the white shrimp Litopenaeus vannamei (9.4–11.3 g) were injected individually with sodium alginate at 10, 20 or 50 μg g−1. No significant differences in THC, DHC and superoxide dismutase activity were observed among the shrimp injected with saline and those injected with sodium alginate at 10, 20 or 50 μg g−1. However, L. vannamei injected with sodium alginate at 20 μg g−1increased its phenoloxidase activity and respiratory burst after 2 days and one day, respectively. L. vannamei injected with sodium alginate at 50 μg g−1maintained a higher phagocytic activity and clearance efficiency to V. alginolyticus after 4 days. In another experiment, L. vannamei which had been injected with sodium alginate, were challenged with V. alginolyticus at 2×105colony-forming units (CFU) shrimp−1and then placed in seawater of 34‰. The survival of shrimp that received sodium alginate at either dose was significantly higher than that of control shrimp at the termination of the experiment (6 days after the challenge). It is therefore concluded that L. vannamei received sodium alginate at 10 μg g−1or more and increased its immune ability and resistance from V. alginolyticus infection.