The toxicity of cadmium and zinc mixtures at concentrations ranging from 0.1 to 10,000 microg/L was investigated against the survival of the free-living cercarial stage of the parasitic fluke Diplostomum spathaceum. Cercariae were exposed to metal mixtures of equal concentration, metal mixtures of unequal concentration, and low-dose pretreatment followed by high-dose exposure mixtures. Under all exposures cercarial survival was increased compared to that achieved with single metals. At exposures with metal mixture of equal concentration in the range 0.1-100 microg/L survival was increased compared to that achieved by controls. The mechanism of metal toxicity and their effects on cercarial survival are discussed.