Intraspecific variation inPolydora ciliata(Johnston) was assessed on the basis of evidence from morphological characters, which were studied by scanning electron microscopy. Differences were observed between populations of P.ciliatawith respect to the setae of the fifth modified segment and in the caruncle. Two other species, namelyP. ligniWebster andP. limicola(Annenkova), were also studied in order to assess interspecific variation. The two forms of P.ciliata, that is boring and non‐boring, were found to be different morphologicall