The marine hoplonemertean Oerstedia dorsalis is considered to be a highly polymorphic species with extensive geographic distribution. We show, based on mitochondrial cytochrome c oxidase subunit I, 16S and nuclear internal transcribed spacer sequences, that there are genetic subsets withing this species. Seventy-one specimens of various colours from different geographic localities (in Europe) were sequenced and analysed using statistical parsimony and Bayesian analysis. Both analyses supported nine major clades. We conclude that O. dorsalis hides different species with geographic resolution. These species, however, appear to be polymorphic as well, and we find no diagnostic features in pigmentation or external characters to separate species within this complex. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 556–567.