Okadaia elegans Baba, 1930, a dorid nudibranch of the family Vayssiereidae (Opisthobranchia: Gastropoda), feeds on spirorbid and serpulid polychaetes. This is the first report of boring by members of the subclass Opisthobranchia. In penetrating the calcareous annelid tubes, specimens of O. elegans bore smooth, round, beveled holes presumably by chemical activity and mechanical rasping. It is hypothesized that the substratum is removed at the site of penetration by the synchronous use of secretion from a gland within the stomodaeum and rasping by cuspidate radular teeth. This hypothesis of the penetrative mechanism is based on observations of the boring behavior of the animals, shape of the borehole, speed of boring, and morphology of the feeding apparatus.