In the Gezira Irrigated Area of Central Sudan, transmission of Schistosoma mansoni was shown to be geographically focal, being concentrated near villages and small settlements. In a study during 1981 and 1982 of the entire area around a typical Gezira village almost 90% of the Biomphalaria pfeifferi snails carrying schistosome infections were found in one minor canal near the village. Sites near smaller settlements some distance from the minor canals yielded few infected snails. The factors which influenced the prevalence of infection in the snails were temperature, turbidity and human contact with the snail habitat. A strategy was proposed for control of Schistosoma transmission through focal and seasonal mollusciciding, health education, chemotherapy, improved water supply and latrine distribution.