14 páginas, 5 tablas, 7 figuras.A new species of Dorvilleidae, Ophryotrocha mediterranea , commensal in the branchial chambers of the deep-water crab Geryon longipes , is fully described and illustrated. It is characterized by the presence of seven pairs of denticulate maxillary plates in the jaw apparatus, and especially the 7th plate which is more elongate, markedly bidentate and situated at a different level. The setae are always spinulate. The gut is annular. The hosts were captured in the Balearic Sea bathyal basin (Western Mediterranean), between 600 m and 1800 m depth. The prevalence of the infestation was 18·87% in male and nil in female hosts. No polychaetes were found in crabs smaller than 41 mm carapace length (49 mm carapace width). No differences were found in the degree of infestation of the right and left branchial chambers. The infestation intensity followed a contagious distribution model. The mean infestation intensity was 4·16 polychaetes per crab. The relative density of the infestation was 0·83 polychaetes per male crab examined and 0·71 per total crabs examinedPeer reviewe
