Sublethal behavioural responses including avoidance and burial rate were compared with 10 d acute morbidity and mortality in bioassays using juvenile Macomona liliana 1 to 3 mm long. The bivalves showed significant movement away from all copper-dosed sediments, with maximal movement after 96 h at 25 mg Cu kg (dry wt)-1. Increasing copper concentrations slowed the rate of burial, and above 15 mg Cu kg (dry wt)-1, most shellfish failed to bury after 90 min. After 10 d exposure, morbidity (defined as inability to rebury) occurred at 15 mg Cu kg (dry wt)-1 and mortality at 30 mg Cu kg (dry wt)-1. In avoidance trials, chlordane had a significant effect on overall movement from 20 gg kg (dry wt) -1, but increasing concentrations reduced movement away from dosed sediment. Chlordane appeared to have no effect on burial rate. Survival was affected by 10d exposure to 400gg chlordane kg (dry wt)-1, with increased morbidity and death. The relative sensitivities of the responses shown by M. liliana to copper were avoidance > burial/morbidity > mortality, with a 6-fold difference in the response threshold between avoidance and mortality. With chlordane, an avoidance response was detected at a concentration ~20-fold lower than that causing morbidity. Behavioural responses of M. liliana have the potential to provide a sensitive bioassay.