Schistosoma infections of Bulinus globosus increased the per capita mortality rate in the laboratory from 0.231 to 0.406 per week. Schistosoma mansoni infections of Biomphalaria pfeifferi increased the per capita mortality rate in the laboratory from 0.055 to 0.177 per week. Calculations based on data from previous work by another author indicate an increase in the mortality rate of B. pfeifferi associated with prepatent S. mansoni infection.