A biogeographic study of bivalve-inhabiting hydroids, Eugymnanthea japonica and the three forms of Eutima japonica, has been made from the intertidal region, or just below it, from Hokkaido to Kyushu (and nearby islands) from 1977 to 1991. Sixty-one species of 47 genera in 23 families of bivalves were examined, and 16 host species of 13 genera in 8 families recorded. Major hosts were 4 species of 4 genera in 3 families. Hydroids were found at 48 of the 126 localities surveyed, including three localities reported by others, 42 of which were on the east coasts facing the Pacific Ocean and the Inland Sea. The four forms differ in range and are distributed parapatrically. Eugymnanthea japonica was mainly found along the Pacific coasts washed by the Kuroshio current, whereas Eutima japonica was in cooler regions. The two species CO-occurred in only three scattered localities. With one exceptional host, Mytilus edulis galloprovincialis, which is introduced, each form was preferentially commensal with a different native host species. The relative abundance and changes of abundance of the two species in two sympatric places are shown. The distributional records of their medusae are also summarized.