The effects of a highly contaminated sediment on life history traits and population dynamics of the nereid polychaete Neanthes arenaceodentata Moore were assessed in a laboratory experiment. Survival, growth and fecundity were measured for one generation of worms exposed to 40 and 10% Black Rock Harbor, Connecticut (BRH) sediment (a contaminated sediment), 100% reference (REF) sediment from central Long Island Sound (the control) and a no-sediment treatment (to follow progress of pairing and reproduction). The BRH sediment did not affect survival or size of adults or number of broods. Number of eggs per brood appeared to be lower in 40% BRH sediment than 10% BRH or REF sediment, but the difference was not significant (1,397, 1,621 and 1,556 eggs per brood, respectively). However, number of larvae per brood was significantly lower in the 40% BRH sediment than in 10% BRH or REF sediment (363, 770 and 850 larvae per brood, respectively). Number of juveniles appeared to be lower in 40% BRH than in the other sediment treatments, although the differences were not significant. Size of juveniles did not differ in the sediment treatments. The finite multiplication rate of population increase, lambda was the same for all treatments. The lack of significant differences but the presence of some trends may indicate that 40% BRH sediment was not a high enough dose to produce significant effects.