The toxicity of sewage sludge to adult and larval Sand shrimp, Crangon crangon, and larval brine shrimp Artemia salina, was studied. Temporal changes in toxicity, ammonia and pH of sewage sludge in seawater were determined. The feasibility of sediment choice bioassays using adult Crangon were examined. The 24 h LC50 of sewage sludge at 20 °C was 0.4% sludge for Crangon larvae, and 10.0% for Artemia larvae. Toxicity to Crangon larvae decreased with decreasing ammonia and increasing pH as sludge decayed over 7 days. In adult Crangon, a dose response with greater variation at < 3% sewage sludge (wet volume in seawater) was found. For adult Crangon at 10 °C, the LT50 at 1% sludge was 80-90 h and 96 h LC50 0.8-1.7% sludge. No clear differences were apparent between 96 h LC5O's in static and 24 h renewal tests. Adult Crangon avoided sediments mixed to concentrations of 6.25% and 10% sewage sludge, but buried at similar frequencies in uncontaminated and 1.57% sludge sediments. In addition to possible chemical effects from ammonia and other sewage sludge components (e.g. metals, organochlorines, hydrocarbons), sludge solids may have physical effects on Crustacea. Bioaccumulation is possible if persistent contaminants occur in the sludge. From estimated field concentrations of sewage sludge, and the results of this and other toxicity studies, lethal effects at sewage sludge disposal sites are believed unlikely. However, further sediment studies (longer-term, life-cycle, bioaccumulation, and mesocosm) are required to evaluate the role of sediment tests in understanding the effects of sewage sludge on benthic Crustacea.