The chromosomes of Platynereis dumerilii (Audouin & Milne-Edwards) are described here for the first time. A modal chromosome number of 2n=28 was recorded, based on counts conducted on metaphase spreads prepared from 24-h-old larvae. The karyotype comprises seven pairs, each of relatively large median (arm ratio, p/q=l.00–0.59) and submedian (arm ratio, 0.59–0.33) chromosomes. Attempts were also made to band the chromosomes using C-banding and silver staining methods. C-band-positive regions were localized on four chromosome pairs (three median, one submedian). A further two chromosome pairs (both median) were observed to have terminal nucleolar organizer regions (NORs). To our knowledge, this is the first time that these banding methods have been successfully applied to any polychaete species. These results are discussed in relation to the karyotypic variation within the class Polychaeta, and the family Nereidae in particular. A stable karyotype consisting of a moderate number of large and morphologically well-differentiated chromosomes, coupled with the ease of culture under laboratory conditions and short generation time, suggests that P. dumerilii is a potentially suitable model for evaluating marine contaminants for genotoxic activity.