Many crustaceans use pheromones to find mates and induce mating behaviours. If pollutants impair the ability to detect chemosensory cues and respond to pheromone signals, they could profoundly affect mating. In a series of laboratory experiments, the effect of copper (0, 0.1 or 0.5 mg Cu(II) per litre for 5 days) on specific components of the mating behaviour of male shore crab Carcinus maenas was investigated, as well as differences in sensitivity between red and green colour morphs. The results show that copper exposure clearly altered the response of C. maenas males to a pheromone stimulus (pre-moult female urine) presented alone, together with a dummy female (a sponge injected with pre-moult female urine) or with a real female. Crabs exposed to the highest copper treatment took more than twice as long to initiate search activity after pheromone introduction and their search behaviour was less directed. When offered a dummy female, male crabs showed decreased pheromone discrimination in both copper treatments. Stroking was the only mating behaviour significantly affected, with a 90% reduction in red crabs in the highest copper treatment. Additionally, crabs of the highest copper treatment more often pinched the dummy female (non-mating behaviour). Finally, male crabs exposed to copper more often pinched pre-moult females and it took about three times longer to establish cradle-carrying. Thus, copper affects the ability of males to detect female pheromones, perform specific mating behaviours and to form pairs.