We examined antimutagenic activity of hot water extracts of twelve edible seaweeds by analyzing the suppressive effect on the SOS response of Salmonella typhimurium induced by direct [furylframide, AF-2 and 4- nitroquinoline 1-oxide, 4NQO] and indirect [3-amino-1-methyl-5H-pyrido- (4,3-b) indole, Trp-P-2 and 2-amino-3-methylimidazo (4,5-f) quinoline, IQ] mutagens. Antimutagenic activities of the seaweed extracts were different from each other against each mutagen used. Among the seaweeds tested, the extract of the brown alga Eisenia bicyclis (Kjellman) Setchell was found to have the strongest antimutagenic activity irrespective of the type of the mutagen used. Total phenolic compounds in E. bicyclis extract was calculated to be 217.9 mg GAE/g dry weight and it was very high in comparison with those of all other seaweed extracts. These experimental results indicated that the hot water-soluble extract of the brown seaweed E. bicyclis has antimutagenic potential and its high phenolic content appears to be responsible for its antimutagenic activity. The E. bicyclis extract was fractionated into polysaccharide fraction and non-polysaccharide one by ethyl alcohol precipitation and the major activity was detected in the non-polysaccharide fraction which exhibited a relatively strong antimutagenic activity against all the mutagens tested. The non-polysaccharide fraction was further separated into high- and low-molecular weight fractions and the former fraction showed much stronger activity than the latter fraction.