Abstract. We have collected evidence that the nominal species, Polydora cornuta Bosc 1802, contains at least three separate species in North America. Specimens of P. cornuta were collected in California, Florida, and Maine, raised in the laboratory, and assessed for reproductive compatibility, genetic similarity, gamete characteristics, and developmental rates. Reproductive crosses between each combination of sex and population revealed variable levels of hybridization at the level of fertilization. Percent fertilization was very low for all combinations (0–7%) except for California females crossed with Florida males (42%). In all interpopulation crosses, fertilized eggs arrested in cleavage and no viable larvae were produced. All pairwise comparisons of the studied populations showed significant differences in multiple reproductive traits. Mitochondrial cytochrome oxidase subunit I DNA sequences revealed large differences between Florida and California worms with a maximum likelihood genetic distance of d=0.860, while Florida and Maine worms were d=0.806, and California and Maine d=0.156. California and New Zealand worms were very similar genetically (d=0.010). These data strongly suggest that populations of P. cornuta in North America comprise a cryptic species complex composed of at least three distinct lineages.