The impact of selective treatment with praziquantel (40 mg/kg) on Schistosoma mansoni prevalence and intensity of infection in two annual follow-up examinations was measured. The target population was the entire rural area of the northern Nile Delta Governorate, Kafr El Sheikh, from which a probability sample was drawn. The sample included 44 villages and hamlets (ezba). Baseline prevalence was determined by the examination of stool by two Kato slides and all infected persons treated and reexamined one year later. Those found infected in the second round were treated and examined again one year later. The prevalence and geometric mean egg count declined across all ages in each follow-up (prevalence: 39.3% (SE +/- 3.3), 28.4% (SE +/- 2.6), and 22.4% (SE +/- 2.3), respectively; and GMEC: 72.9 (SE +/- 7.3), 52.5 (SE +/- 4.5), and 41.9 (SE +/- 2.4), respectively). Reduction in prevalence varied considerably by village and ezba and was strongly related to the proportion of the village or ezba population that was infected and treated (r2 = 0.29). This latter observation provides a rationale for the maximum application of chemotherapy in the endemic Nile Delta community.