Abstract: 400 cases of schistosomiasis japonica from villages of different endemicity were randomly divided into four groups and treated with four different dosages of praziqu antel, namely, 60, 50, 40 and 30mg/kg of body weight. The drug was given twice in one day. Identical placebo tablets making up a total of 60mg/kg were used. The reactions were generally mild. At six-month follow-up, the stool negative rates were 79.8%(79/99), 71.7% (71/99), 78.8% (78/99) and 70.1% (68/97) respectively for the four groups. No significant differences were found among them. In villages where Oncomelania snails were very few the negative hatching rates in the aforementioned four groups were 89.1% (49/55), 91.1% (51/56), 88.9% (48/54) and 81.8% (45/55), while in villages with more snails the cure rates were 68.2% (30/44), 46.5% (20/43), 66.7% (30/45) and 54.8% (23/42) (P0.01), indicating that reinfection probably occurred in the latter area. The authors suggest that praziquantel with a lower total dose of 30 to 40mg/kg might be preferable for the mass treatment of chronic schistosomiasis.