The overall prevalences of infection with Schistosoma mansoni, S. haematobium or both species among 6122 children from 27 schools in the former White Nile Province of Sudan (now divided into the Kosti, El Getaina, El Gebelein and El Duem Provinces) were 10.1%, 21.4% and 4.5%, respectively. Geometric mean egg counts in the pure S. mansoni and pure S. haematobium infections were 97.7 eggs/g faeces and 12.9 eggs/10 ml urine, respectively. There were marked differences in levels of endemicity between provinces and also between schools within each province. School-specific prevalences and intensities of infection were positively correlated, both for S. mansoni and for S. haematobium. Prevalences and intensities of infection were generally similar in male and female subjects and in the different age groups into which they were split (i.e. 5-9, 10-14 and 15-19 years). Community-based surveys of all age groups in two villages showed typical patterns of infections with S. mansoni and S. haematobium, with indices peaking in those aged 10-19 years, both in males and females. Observations on snail-related aspects of transmission revealed that transmission of both S. mansoni and S. haematobium was highly focal and also seasonal, taking place during the hot, dry and post-rainy seasons.