Callithamnion callophyllidicola Yamada was collected in abundance from the Oki Islands in the Sea of Japan. Its morphology was examined detailedly and is compared with previous reports made by Segawa (1942, 1949) and Kawashima (1960) on this species and that of its allied 3 species which have been assigned to the genus Aglaothamnion, such as C. tripinnatum, C. decompositum and C. oosumiense comb, nov. The result of this study suggests that the material used for Kawashima's study is probably a distinct species.