3 × 10−6M praziquantel fails to completely paralyse miracidia and cercariae in a short time but they are not infective when maintained in the solution. 3 × 10−5M praziquantel prevents infected snails shedding cercariae but does not kill daughter sporocyts or developing cercariae. As the action of praziquantel on adult worms is not blocked by 10−2M mecamylamine, pempidine or carbachol, but is reduced by calcium deplection, it is suggested that praziquantel may act by permitting calcium influx to muscle cells causing them to contract.