AbstractThree single‐species bioassays (Microtox,Selenastrum capricornutum, andPanagrellus redivivus) and a test using microbial communities developed on artificial substrates were used in a series ofin situand laboratory tests evaluating the ecotoxicological hazard of contaminated sediments at two sites on Lake Michigan: Waukegan (Illinois) Harbor and The Chicago Area Confined Disposal Facility Study. In the single‐species tests, exposure to elutriates of contaminated sediments significantly inhibited bacterial luminescence, algal photosynthesis, and nematode survival and growth at polluted stations, while elutriates from control stations did not. The battery of three tests is a promising screening tool for in‐place pollutants.Protozoan species richness and protozoan phototroph abundance were inhibited by elutriates from contaminated sites, but the abundance of heterotrophic protozoans was enhanced by sediment elutriates from some stations. Microbial community photosynthesis was significantly inhibited by most sediment elutriates, while community respiration was often stimulated; thus, functional responses paralleled the structural changes. Overall, the results of the microbial community tests were consistent with expected patterns of toxicity at the two sites on Lake Michigan.In general, single‐species test results agreed with the community bioassays. Although community tests may be more realistic than single‐species bioassays in predicting the impact of sediment contamination on actual ecosystems, caution must be exercised in interpreting th