Abstra~t-.Sedi~ent reworking ra!e, mortality and organism dry weight were measured for Stylodri lus hermg1anus m laboratory microcosms. The experiments were designed to identify potential population-specific response differences to mixed (stirred to obtain a more uniform particle size dis tribution over depth) and unmixed (passively settled) microcosm sediments. Lake Michigan sedi ments and worms were collected offshore Benton Harbor, Michigan and Grand Haven, Michigan. The mixed Benton Harbor sediments were toxic to S. heringianus collected from Grand Haven, whereas there were no significant differences in measured responses between mixed and unmixed sediment microcosms for Grand Haven-collected worms exposed to Grand Haven sediments or Ben ton Harbor-collected worms exposed to Benton Harbor sediments. Note that the mixing of sedi ments resulted in increased availability of contaminants sorbed to the fine sediment fraction. Because contaminant and oligochaete population density data suggest that Grand Haven sedi ments are less contaminated, the population-specific response suggests that S. heringianus may adapt to the low level long-term stressful conditions (chemical or otherwise). Results also suggest caution and consideration of the history of test organisms in the design and interpretation of toxicity tests.