A study to ascertain the age and sex distribution and the epidemiology of Schistosoma haematobium infection in a hyperendemic area of Ibadan, Nigeria, showed that of the preschool children (usually less than 6 years old) in the Odo Ona area of Ibadan, 17.4% were passing S. haematobium ova in their urine. The earliest age of infection was 2 years; of those 2 through 6, 20% were infected. Some children passed as many as 800 ova per 10 ml of urine without visible hematuria. The rate of infection was 85 effective contacts per 1,000 children per year, and in the homes bordering the river, the infection rate was significantly higher than that for the whole area. The lowest rates of infection, however, were among those living in houses with private water taps.