Biomphalaria glabrata 2 to 12 mm in diameter were exposed to 1 miracidium of Schistosoma mansoni and after 15, 30, or 60 days were reexposed to the homologous strain. The criterion of superinfection was the subsequent escape of cercariae of both sexes. Opposite sex superinfection was acquired by 45 of 106 snails of various sizes, all of which were in the group reexposed at 30 days. The observations confirm those of Kagan and Geiger and suggest that in the schistosomes a maximizing of the number of intermediate hosts with larval populations of both sexes may be an adaptation having a high species survival value.