89 people from 6 to 60 years old living in two villages in the valley of the Oyako near Bamako (Mali) infected with S. mansoni, were treated with a single dose of 40 mg/kg bwt. 2-cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino [2,1-a]isoquinolin-4-one (praziquantel, EMBAY 8440, Biltricide). Side effects were recorded, a clinical and parasitological examination was effected 5 months later. If the results on efficacy can be confirmed, considering the ease of administration and the good acceptance by the patients praziquantel will be recommended for mass campaigns in Mali, where S. mansoni and especially S. haematobium are rapidly spreading following improvement of the water supply.