A survey of sediment toxicity was carried out by NOAA's National Status and Trends Program in the coastal bays that surround Long Island Sound in New York and Connecticut. The survey objectives were to determine the spatial distribution and severity of toxicity, and to analyze the relationships between toxicity and chemical contamination in the sediments. Sediment samples from three stations in each of 20 coastal bays and one Long Island Sound site were tested for toxicity with three independent protocols: (1) a 10-day amphipod survival test of the whole, solid-phase sediments with Ampelisca abdita, (2) a 48-hour exposure of clam larvae, Mulinia lateralis, to sediment elutriates, with normal development and survival as the endpoints, and (3) a microbial bioluminescence test (MicrotoxR) using solvent extracts of the sediments. Separate samples from these same stations were analyzed chemically for a broad suite of potentially toxic contaminants, including heavy metals, polynuclear aromatic hydrocarbons (PAH), chlorinated pesticides and polychlorinated biphenyls. Additional sediment samples were obtained from up to six additional stations in a few of the coastal bays; these samples were examined only for heavy metals contamination and the data are included in an appendix to this report.The survey results indicate that sediment toxicity is widespread in the coastal bays of Long Island Sound. Significant toxicity was indicated for the sediments from at least one of the stations in each of the 20 coastal bays sampled in this survey. Manhassett Bay, Oyster Bay, and Little Neck Bay, New York were the three most toxic bays, respectively, as indicated by the incidence of significant toxicity from the three tests on samples from three stations. Only 11 of the 60 stations showed no significant toxicity in any of the three tests. Branford Harbor and the Connecticut River were indicated as the least toxic bays by this approach. About one-fifth of the total area (79.1 km2) sampled within the 20 embayments was indicated as significantly toxic by all three tests (survival of amphipods and larval bivalves, and MicrotoxTM).