We have studied the repellent activity against the blue mussel, Mytilus edulis, which is one of the gregarious fouling organisms causing serious problems to ships and fishing nets. In the previous paper, we reported the isolation and identification of one of the flavonoids, kaempferol 3-O-(2^6"-di-O-(E)-p-coumaroyl-p-D-glucopyranoside), from Quercus dentata, and of rhaponticin 6"-O-(E)-p-coumarate from Eucalyptus rubida as repellent compounds against M. edulis. Since these compounds contain phenolic groups, we expected the presence of the repellents in bark containing many phenolic substances. We found that methanol extracts of the bark of Prunus jamasakura (Japanese name "Yamazakura") showed the most potent repellent activity against M. edulis among the several kinds of bark extracts tested (Table I). Here, we report the isolation of the active substances, together with their repellent activity against the blue mussel.