Epidemiological and experimental studies indicate that heterologous immunity resulting from exposure to bovine and other animal schistosomes might be of some importance in determining the relative severity of schistosome infection in man in different parts of the world. The reduction in the severity of a disease process, resulting from exposure to relatively non-pathogenic infections, has been called "zooprophylaxis".The observations recorded in the present paper on the interaction in mice of Schistosoma bovis, S. mattheei and S. rodhaini with S. mansoni provide further evidence of this phenomenon. The most potent immunizing effect was produced with S. bovis and S. mattheei, which resulted in a reduction in the expected S. mansoni egg load of 74% and 85.7%, respectively.It is suggested that zooprophylaxis may be an important factor in limiting the severity of schistosomiasis in areas of Africa where bovine and other animal schistosomes are prevalent. The relatively more severe nature of the disease in Egypt and Brazil may be in part due to the absence of concomitant transmission of bovine schistosomes.