Ultrastructural observations were made of changes in the tegument and reproductive organs of Schistosoma japonicum and S. mansoni from ICR mice after treatment with praziquantel (PZQ), levo-PZQ, and dextro-PZQ at a single oral dose of 500 mg/kg body weight. No marked difference in types and extent of lesions of the tegument of S. japonicum was found between the compounds regardless of the time of worm recovery after treatment. This was equally true of S. mansoni. Degeneration of the testis, ovary, and vitelline gland of S. japonicum was more prominent in worms administered PZQ and levo-PZQ than in those receiving dextro-PZQ. In S. mansoni, extensive regression of the reproductive organs was observed in male and female worms treated with PZQ and dextro-PZQ, while no serious damage was seen in worms treated with levo-PZQ.