The chromosome complement for the marine worm, Neanthes arenaceodentata, consists of nine pairs; one pair has a median centromere, seven pairs have submedian centromeres, and one pair is polymorphic with either a subterminal or terminal centromere. A technique for studying polychaete chromosomes is described. This species was selected as a model for studying cytogenetic effects of toxicants in marine environments because it has a suitable karyotype, is easily cultured, and as a benthic organism it is representative of an ecologically important group of organisms.