Pseudomonocelis ophiocephala (Schmidt, 1861), a mesopsammic proseriate, is common in a variety of shallow water habitats in the Mediterranean. The genetic relationships between morphologically indistinguishable populations across the Mediterranean were surveyed by means of combined allozyme and Random Amplified Polymorphic DNA (RAPD) analysis. Genetic distances, UPGMA cluster analysis and F-statistics based on 27 allozyme genetic loci and 68 RAPD primer fragments were consistent in showing that the taxon Pseudomonocelis ophiocephala is a complex of sibling species, consisting of four taxonomic units. The four species differ in distribution and habitat: sibling A is widespread in lower intertidal habitats of the Mediterranean, in well-sorted, medium- to coarse-grained sand; siblings B, C and D show a restricted distribution (Corsican–Sardinian region and Elba Island, west and east coast of Greece, respectively) in low-energy marine habitats. Given that the species of the complex are morphologically indistinguishable, the type material is absent, and that there are two siblings in the type locality (Corfu Island, Greece), a neotype is designated for P. ophiocephala. The three further siblings are named; species descriptions are based on non-morphological characters (karyotype, allozymic and RAPD patterns). Distributions and reconstruction of phylogenetic relationships based on allozyme data suggest that both allopatric and ecological speciation have played a role in the evolutionary history of the species complex. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87, 553–576.