The distribution of urinary schistosomiasis in schoolchildren on Zanzibar Island (Unguja) was surveyed in May 2001 to test a potential correlation with the distribution of snail species of the Bulinus africanus group and to record contemporary baseline epidemiological data. Quasi-random samples of 40 schoolchildren of mixed sexes were selected from each of 10 schools. Schistosoma haematobium infections were detected upon the basis of micro-haematuria with subsequent confirmation by microscopy examination for schistosome eggs. At the time of urine collection, each child was interviewed with a suite of 12 questions prepared as a standardized questionnaire. Total prevalence of urinary schistosomiasis (known locally as kichocho) was 12% although schistosome infections were absent in 5 schools. Schools located west of 39 degrees 19'E and north of 6 degrees 10'S harboured nearly all of the infections; the highest prevalence (55%) was found at Kinyasini where many B. globosus habitats occur nearby. The general level of understanding of kichocho was low (24%) and individual self-diagnosis was poor (sensitivity, 8.5%; specificity, 85%). Grouped freshwater-contact patterns of schoolchildren differed significantly between schools and correlated well with prevalence of infections within schools. Across the island the area of active transmission of S. haematobium to humans appears confined within the distribution of B. globosus. There was no epidemiological evidence to suggest any involvement of B. nasutus in local transmission, confirming previous laboratory findings. In areas where B. globosus occurs, targeted snail control should be considered, to reduce schistosome transmission.