Abstract: Human schistosomiasis or bilharziasis is a water borne disease of man caused by trematodes of the genus Schistosoma and transmitted by fresh water snails. It is one of the most widespread parasitic infections being second only to malaria in its socioeconomic and public health importance in tropical and subtropical areas. Intestinal schistosomiasis is endemic only in areas with certain physical characteristics and risk factors. A 2 years study was conducted in the Budalangi endemic focus of Western Kenya to determine prevalence and intensity of infection and reinfection in the risk population with a view of instituting a deworming programme in primary schools. Stool samples collected from 972 school children aged 5-14 years in 18 randomly selected schools were examined for ova of S. mansoni. A questionnaire was used to obtain demographic information and to quantify exposure to infested surface waters. The mean prevalence of infection in all schools (32.1%; range 0.0-65.0%) was significantly higher in females (58.0%) than in males sampled (42.0%) (χ²; p<0.05). Intensity of infection was predominantly light with 65.1% of the infected population harbouring GMEC 1-99 epg. The prevalence of infection significantly varied with age groups (χ²-test; p<0.05). The 8-11 years age group accounted for 49.5% of the potential contamination of contact sites by ova of S. mansoni. The prevalence and intensity of infection significantly dropped (Studentised t-test; p<0.05) after treatment with praziquantel coupled with training in primary hygiene. Proximity to the lake shoreline (R² = 0.89, p<0.05), contact with lake water (χ²; p<0.05) and specific water-related activities including swimming (OR = 1.70; 95% CI = 1.03-2.81), fishing (OR = 3.23; 95% CI = 1.70-6.15) and washing clothes in the lake (OR = 2.07; 95% CI = 1.27-3.38) were all associated with high risks of infection. The studies showed that there was a continuous low level of transmission of S. mansoni in the study area among the various exposure groups attributable to inevitable contact with permanent water bodies which the vector snail used for breeding. The level of infection in the human population determined in these studies could be used for planning and implementing combined mass treatment of people who reside in the study area in order to enhance control of the parasite.