Relative toxicity of sewage sludges from six treatment plants was determined by the LC50 of sludge-sediment mixtures to the infaunal marine amphipod, Rhepoxynius abronius. LC50s were measured as the increase in the percent total volatile solids (TVS) of the mixture due to the addition of sludge required to kill 50% of the amphipods during a 10-day exposure. LC50s ranged from 2.83% TVS addition for sludge from the small, domestic community of Waldport, Oregon to less than 0.1% TVS addition for metropolitan, more industrialized sources in Los Angeles, California. The toxicity of the sludge-sediment mixtures is attributed primarily to chemical contamination rather than organic enrichment. Rank correlations between toxicity and specific chemicals were usually not statistically significant, indicating that different combinations of stresses involving multiple or unmeasured factors were probably responsible for observed effects.