Computer assisted sperm analysis (CASA) has been used to analyse the effects of cadmium and zinc on the motility of fish sperm treated either for 24 h after partial dilution in an extender, or during the final dilution step on the microscope stage during video recording. The progressive motility of catfish sperm, which was not affected by storage for 24 h, was decreased after exposure to 100 ppm cadmium or 2000 ppm zinc in extender for 24 h. Only 1000 ppm cadmium or 2000 ppm zinc had any effect on motility when added to sperm at the final dilution stage. These results suggest that concentrations of cadmium which have been found to be present in fish gonads as a result of bioaccumulation from the water or diet may decrease the motility of sperm during maturation or storage in the testis. Cadmium toxicity was significantly decreased in the presence of an equal amount of zinc, suggesting an equilibrium between zinc and cadmium at binding sites in the sperm. Computer assisted sperm analysis is shown for the first time to provide a rapid and quantitative measure of the effects of environmental pollutants on the quality of fish sperm. It is suggested that the methodology described may form the basis for a standard toxicity test to assess the potential hazards of environmental pollutants on reproduction.