Seawater is used as a readily available heat exchange medium in steam condensers and LNG regasifiers in coastal power plants and factories. Hydrogen peroxide is often used as an antifoulant in such systems or as a veterinary product for aquaculture. However, ecotoxicological studies of hydrogen peroxide on a variety of marine organisms, especially for common coastal marine species are insufficient unlike those of chlorine which is another antifoulant. Therefore, we studied the biological effect of hydrogen peroxide on five phytoplanktons (Heterocapsa triquetra, Rhodomonas salina, Tetraselmis sp., Skeletonema marinoi-dohrnii complex and Chaetoceros didymus), seaweed gametophytes (Undaria pinnatifida), and crustacean zooplanktons (marine harpacticoid copepod Tigriopus japonicus and larvae of the barnacle Amphibalanus amphitrite), and compared with that of residual chlorine. Test results indicated that the lowest no observed effect concentrations (NOEC) of hydrogen peroxide and residual chlorine were estimated to be 1.05 mg/L and less than 0.1 mg/L for all the organisms used in this study, respectively. The difference between the NOECs of hydrogen peroxide and residual chlorine was considered to be due to their different action mechanisms.