Hot-water extracts from 58 species of marine algae (5 green, 33 brown and 20 red algae) were evaluated for their ability to enhance the resistance of carp (Cyprinus carpio) against Edwardsiella tarda infection. The extract was injected intraperitoneally to carp at a dose of 50 mg/kg on days 1 and 4. On day 7 the fish were challenged with E.. tarda (3×10^8 CFU/100g) and the survivors were recorded daily for 7 days. Among the algae tested, the extracts from 6 brown and 8 red algae significantly prolonged the survival time of fish, but those from green algae were not effective. In addition, the extracts from 4 brown algae (MyeloPhycus simplex, Undaria pinnatifida, Laminaria japonica, Sargassum autumnale) and 4 red algae (Porphyra yezoensis, Gloiopeltis furcata, Carpopeltis affnis, Hypnea charoides), which were effective to carp, also increased the resistance of yellowtail (Seriola quinqueradiata) against Streptococcus sp. infection.