Praziquantel is a new chemotherapeutic agent with high activity against juvenile and adult cestodes. The juvenile cestodes tested include Hymenolepis nana, Cysticercus fasciolaris, C. pisiformis, C. tenuicollis, C. bovis. The fully effective doses range from 25–100 mg/kg host. All adult tapeworms tested (Taenia pisiformis, T. hydatigena, T. ovis, T. taenaiformis, Dipylidium caninum, Mesocestoides corti, Echinococcus multilocularis, E, granulosus) of dogs and cats and several Moniezia species of sheep were killed by doses up to 5 mg/kg host. Praziquantel is effective in vitro at very low concentrations. First studies on the mechanism of action in H. diminuta have shown that praziquantel acts on the carbohydrate metabolism of the parasite.