Two different regiments of praziquantel, 40 mg/kg in a single dose and 60 mg/kg in two divided doses administered 6 hr apart for the treatment of S. mansoni infection, were compared at two villages of Beheira Governorate. All residents of both sexes between 5 and 50 yrs old from two rural communities (Farshout and Om El-Laban) participated in this study. The prevalence of S. mansoni infection was 58% at Farshout and 69% at Om El-Laban. Infected subjects received 60 mg/kg of praziquantel in two divided doses 6 hr apart at Farshout and 40 mg/kg in a single dose at Om El-Laban. Two months post-treatment, better cure rates (96% vs 85%, P < 0.001) were achieved with 60 mg/kg of praziquantel. Twelve months post-treatment the prevalence of infection (45% for both villages) increased, but remained lower than before treatment. This may indicate that transmission continued from cercariae transported by water from nearby hyperendemic areas. The prevalence and intensity of infection were higher in the younger (5-19 y) than in the older (20-50 y) age group both before treatment and one year post-treatment. The same applies to the incidence and reinfection of the treated subjects. We recommend that 60 mg/kg in two divided doses six hours apart be used for school children. For logistical reasons, the single dose, 40 mg/kg regimen, should be used for mass treatment. For better control of disease, mass chemotherapy with praziquantel as suggested above is essential for the whole endemic region rather than only for a focal area.