Previous studies on heterologous immunity in mice have indicated that Schistosoma bovis and S. mattheei could be used to limit the severity of infection resulting from subsequent challenge by S. mansoni. These observations have now been extended to study the immunizing effect in rhesus monkeys of both S. mattheei and S. bovis.The bovine schistosomes were shown to be relatively non-pathogenic in rhesus monkeys. Immunization with 1000-2000 cercariae resulted in a marked reduction in the pathogenic effect of subsequent challenge with S. mansoni. This effect was demonstrated by a decrease in the worm load and tissue egg densities in 10 immunized monkeys as compared with 5 control animals.There was no correlation between fluorescent antibody titres and the intensity of infection or the degree of acquired immunity. There was a cross-reaction between S. mansoni and the bovine schistosomes.It is suggested that natural heterologous immunity (zooprophylaxis) may be of considerable epidemiological importance in determining the severity of schistosomiasis in man.