Abstract: When ³H praziquantel was given orally to mice infected with Schistosoma japonicum at different developmental stages, less radioactivity was detected in the tissues of d_o (3 h) schistosomula by autoradiography. Thereafter, the radioactivity in the tissues of schistosomules increased gradually and reached a higher level in d₂₁ schistosomula which was similar to that of adult worms (d₂₈ and d₃₅).In d₀ and d₃ schistosomula the radioactivity distributed primarily in parenchyma. Up to d₁₁ schistosomula,some radioactivity appeared in tegument and increased gradually thereafter. In d₂₁ schistosomula more radioactivity was observed in tegument. Since only d₀, d₂₈, d₃₅ and d₄₂ worms can be killed by praziquantel, the susceptibility of schistosomes of different developmental stages to praziquantel may not depend on the amount of the drug entered into the worm body.