Short-term (6 h) behavioural responses ofPerna pernaexposed to different concentrations (0,2, 0,4, 0,6, 0,8 ppm) of copper in sea-water (35‰) have been investigated.Pernadoes not respond to copper until exposed to 0,2 ppm. The initial response to all concentrations is a sharp adduction followed by a period of rapid valve movements. The animals never completely close their valves; nevertheless the heart rate gradually decreases suggesting isolation of tissues by use of the mantle edge. The responses exhibited byPernaare compared to those of other species.