Abstract: Half an hour after oral administration of pyquiton (praziquantel) 300mg/kg to infected mice the tegument of schistosomes removed from the animals showed edema and vacuolization in various degrees followed by rupture of vacuoles and formation of flocculous materials attached to the damaged surface of the worms. In gynecophoric canal Of male schistosomes the flocculous materials fused to form a large mass. Six hours after medication numerous leucooytes were attached to damaged tegument of the worms. The penetration of the leucocytes into the worm body occurred 12 hours later. The results indicate that the appearance of pathological changes of the worm tegument caused by pyquiton was earlier in male worms than in female worms. The relationship between the tegument damage and the penetration of the host leucocytes into the worm body was discussed.