Sediment toxicity, contamination, and macrobenthic community structure were examined in 1980 at seven stations along a pollution gradient from the Los Angeles County Sanitation Districts' sewage outfalls on the Palos Verdes Shelf, CA, to control conditions in Santa Monica Bay. Sediment toxicity was determined by laboratory bioassays with the phoxocephalid amphipod, Rhepoxynius abronius. Distribution and abundance of the macrobenthos were generally consistent with the Pearson-Rosenberg model and the Bascom-Mearns-Word quantitative classification of macrobenthic assemblages. Species richness, density, and biomass increased greatly in areas of moderate sediment organic enrichment, but decreased to or below control conditions near the outfalls. The Infaunal Index of changes in benthic community structure in response to organic enrichment increased with distance from the outfalls. Dominant species changed from the opportunistic polychaete, Capitella capitata, near the outfalls; to the clam, Parvilucina tenuisculpta, and the polychaetes, Mediomastus californiensis and Tharyx sp. A in areas of moderate organic enrichment; to the brittlestar, Amphiodia urtica, at the control station. Sediment toxicity was significantly greater than control levels at the three stations closest (≤3 km) to the outfalls. There were significant increases in the concentration of most sediment contaminants and significant decreases in the richness and abundance of the benthos at stations where sediment was actely toxic to Rhepoxynius abronius. Organic enrichment and anaerobic sediment conditions appear to be the dominant anthropogenic influences on the macrobenthos of the Palos Verdes Shelf. Toxicity caused by chemical contamination may contribute to the absence of amphipods near the sewage outfalls.