Abstract:The total haemocyte count (THC), phenoloxidase activity, and respiratory burst were examined when the white shrimp Litopenaeus vannamei (10.42±1.39 g) were immersed in seawater (34‰) containing hot-water extract of brown alga Sargassum duplicatum at 100, 300 and 500 mg l−1, or injected with hot-water extract of S. duplicatum at 2, 6, 10 and 20 μg g−1. These parameters increased significantly when the shrimp were immersed in seawater containing hot-water extract at 300 and 500 mg l−1 after 1 h, or when the shrimp were injected with hot-water extract at 10 and 20 μg g−1 after 1 day. L. vannamei that were injected with hot-water extract at 6, 10 and 20 μg g−1 had increased phagocytic activity and clearance efficiency to V. alginolyticus after 1–6 days. In another experiment, L. vannamei which had been immersed in seawater containing hot-water extract at 100, 300 and 500 mg l−1, or injected with hot-water extract at 2, 6, 10 or 20 μg g−1 were challenged with V. alginolyticus at 1×106, or 1.4×106 colony-forming units (cfu) shrimp−1, and then placed in seawater. The survival of shrimp that received hot-water extract at either dose was significantly higher than that of control shrimp after 2 days, as well as at the termination of the experiment (6 days after the challenge). It is therefore concluded that L. vannamei that were immersed in hot-water extract of S. duplicatum at 300 mg l−1, or the shrimp that were injected with hot-water extract at 10 μg g−1 or less had increased immune ability as well as resistance to V. alginolyticus infection.