The toxicity of cadmium and zinc at concentrations ranging from 0.1 to 100 microg/l was investigated against the activity of Diplostomum spathaceum (Rudolphi, 1819) cercariae. Over a 24 h exposure period a significant reduction in cercarial activity occurred in solutions of cadmium, zinc, and a mixture of cadmium and zinc at all concentrations. Reduced cercarial activity also occurred in all toxicant solutions compared with controls after only 6 h exposure indicating that cercariae were vulnerable during the period of maximum cercarial infectivity (0-5 h). The mechanisms of metal toxicity and their importance to parasite transmission are discussed.