We determined the prevalence of urinary schistosomiasis in preschool children below the age of 5 years in three settlements around Oyan Reservoir in Ogun State, Nigeria. Of 209 children screened, 150 (71.8%) had an infection, with no significant difference between males and females; 42.9% of infants were infected. Both prevalence and intensity of infection increased significantly with age (P < 0.005). Most (62.7%) infections were light (<50 eggs/10 ml urine). A 17.7 percentage of the children had visible haematuria, which increased with age (P < 0.005). Focus group discussions (FGDs) with adult men and women revealed that infection in preschool children was primarily because of exposure occasioned by the mothers' domestic (washing and bathing) and occupational (fishing) activities, while older children could go swimming on their own. Although the participants claimed that using a different water supply may not be effective in combating the disease, as their entire existence was tied to the reservoir, we propose that health education geared towards changing behaviour and attitudes is necessary. As preschool children are a source of both contamination and transmission, control programmes must take them into account.