Techniques for recovering Schistosoma mansoni from the skin, lungs, and liver have enabled us to trace the attrition of a challenge infection in immune mice. Two separate phases of attrition were demonstrated. In mice which had developed immunity following exposure to 20 cercariae a major phase of attrition occurred between days 6 and 13, after the parasites had left the lungs, and only in mice which had been infected for 12 weeks or more did a minor phase of attrition occur in the skin. In contrast, mice which had been immunized by exposure to highly irradiated (20 kr) cercariae destroyed the majority of their reinfecting population in the skin. These results suggest that there are two distinct mechanisms of immunity to S. mansoni in mice. This view was strengthened by experiments in which both types of immune mice were challenged intravenously with 5-day-old lung schistosomula (a challenge which bypassed the skin). While the immunized mice were susceptible to the lung worm challenge, the infected mice were substantially resistant to it.