Like so many other serpulid genera, Ditrupa has a history of taxonomic confusion, even to the extent that the worm tubes have been included in the Mollusca by some workers. In recent publications, most authors follow Fauvel (e.g. 1953) and recognise a single species, Ditrupa arietina O.F. Müller, 1776, worldwide, despite the fact that descriptions of this species were based on European and Atlantic material. A re-examination of type specimens and study of a broad range of samples from the Indo-Pacific now justifies the recognition of a separate species, Ditrupa gracillima Grube, 1878, from this region. It shows extreme variation, at least partly related to depth: from 100 m downwards an ecophenotype prevails, previously described as D. arietina var. monilifera.