Abstract: F-30066, a derivative of 5-nitrofuran, has been recently demonstrated as an effective chemotherapeutic agent in experimental schistosomiasis. The aim of this investigation is to elucidate its in vitro and in vivo mode of action on schistosomes from the histochemical point of view. Three pairs of adult schistosomes, obtained from guinea-pigs artificially infected with cercariae shed from laboratory infected Oncomelania snails, were incubated at 37℃ in 6ml of glucose-Tyrode's medium containing 50γ/ml of F-30066 for 6 or 12 hours. Six hours after incubation, the worms appeared paralytic and their suckers lost the power of attachment. The reaction of PAS for glycogen in the parenchyma of male worms generally became weak, but the subcuticular muscles were still rich in glycogen. No observable change of glycogen contents in the parenchyma of female worms was seen. After 12 hours, glycogen in the parenchyma and the subcuticular muscles of both sexes disappeared almost completely, though some feeble PAS reaction might be occasionally detected in a few female worms. Schistosomes incubated under the same condition for control, however, moved actively and glycogen was richly distributed in their parenchyma and muscles. In the liver sections of mice autopsied 3 days after administration of F-30066, numerous worms were found blocking the portal veins, and they showed negative reaction for PAS. No visible in vitro and in vivo effect on other histochemical substances such as nucleic acids, phosphomonoesterases, and lipid could be observed.