Four human volunteers were exposed to 400 cercariae, and one to two batches each of 400 cercariae, of the Formosan strain of S. japonicum. Acute abdominal pain, positive intradermal reactions, eosinophilia and histopathologic changes in the liver tissue similar to those produced by unisexual infection of male worms were found in due time after the exposure. Using the sedimentation method, daily stool examinations were negative for S. japonicum eggs throughout the study period of 204–336 days and liver biopsies showed negative findings of schistosome eggs and pigment. It is concluded that the Formosan strain of S. japonicum develops for a short period in the viscera of man but does not reach maturity. It must be regarded as a non-human, zoophilic strain.