The behaviour and physiological responses of Mytilus edulis L. to increased concentrations of copperare dependent on the absolute copper concentration present and not the rate of increase of concentration. The sequence of behavioural responses isolating the animal from heightened sea-water copper concentration is similar to that observed in response to lowered salinities. The effects of copper onthe oxygen consumption of intact and ‘propped open’ and animals whose posterior adductor muscles havebeen severed suggest that only measurements of animals in the latter state give a true reflexion of the effects of copper on the oxygen consumption of the tissues of Mytilus. The effects of increased sea-water copper concentrations on the filtration and ventilation activity of mussels are assessed, but consideration of the data in terms of Dejour's equation of gaseous exchangesuggest that indirect measurement of ventilation activity gives rise to differing results. The adaptive advantage of theresponse of Mytilus to heightened sea-water copper concentrations is considered.