Previous studies have shown that when cattle are exposed to cercariae of Schistosoma mansoni they develop considerable resistance to subsequent challenge with S. mattheei. The present study showed that when sheep are immunized with S. mansoni cercariae they too develop a marked resistance to subsequent challenge with S. mattheei. A small proportion of the immunizing cercariae reached maturity and a few viable S. mansoni eggs were found in the faeces of the sheep. The results of this experiment provide further evidence that heterologous immunity may be of significance in limiting the severity of schistosomiasis in both man and domestic animals in areas where animal and human schistosomes occur together.