Abstract: The beef tapeworms from patients with T. seginata infections treated by pyquiton became spasmodically contracted and immobilized. Histological changes showed contractions of muscles and spermatozoa as well as wrinkling, vacuolization, swelling and breaking of the tegument, subtegument and parenchymal organs, which finally led to the disruption of the structures of the whole worm. Histochemical observations indicated that the glycogen granules within the parenchyma became disintegrated or disappeared; the activities of AKP and ACP of tegument markedly decreased; and the granules of DNA and RNA dissociated and reduced, while the reactions shown by mercurobromophenol blue method for basic proteins in muscles and reproductive cells were stronger than those found in the control. The mechanism of tapeworm expulsion by pyquiton was tentatively discussed.