Abstract: When Schistosoma japonicum cercariae were exposed to praziquantel 1 or 10 fig/ml in dechlorinated tap water, the tail of cercariae was separated from the body within 1 minute or more. The cercarial bodies became swollen and dead within 2-4h. If the cercariae were exposed to praziquantel for O.Sh and then transferred to serum-HBS, the tail-less cercariae promptly resumed their normal movement. They were characterized by water sensitivity, depletion of penetration gland contents, complement insensitivity, negative "Cercarien-Hiillen Reaktion" and loss of normal cercarial shape (Table 1, Plates 1-6). These tailless cercariae induced by praziquantel satisfied the criteria of postpenetration schistosomula.