With a method by which the penetration of cercariae of Schistosoma mansoni can be quantitated separately from their subsequent development into worms, a comparative study was performed in the hamster, mouse, guinea pig, rabbit, and rat. Cercarial penetration was almost identical in the five species of rodent. Four weeks after exposure, in relation to the number of cercariae that penetrated, the maturation of the worms was 50% in the hamsters, between 36 and 39% in the mice, guinea pigs, and rabbits, and 26% in the rats. The infection remained constant in the hamsters and mice for at least 16 weeks, but diminished gradually in the rabbits and guinea pigs. The rats rapidly eliminated the schistosomes.