Chemotherapy of all infected individuals, using praziquantel 40 mg/kg in a single dose, was the initial component of the recently introduced control programme against Schistosoma mansoni-infections in Ngamiland, Botswana. To evaluate the effect of praziquantel in Ngamiland, 81 children were selected at random from three strata of intensities of infection. Six weeks after treatment a duplicate of Kato fecal thick smear was made. When eggs of S. mansoni were found, a new stool sample was collected and a hatching test performed. Only in case of a positive hatching test was the child considered to be infected: Cure rates between 78.6 and 90.0%, and reductions in egg output among noncured between 84.6 and 98.0% were found.