The geographical location and physical features of the island of Leyte in the Philippines, where bilharziasis is endemic and where the studies reported here were conducted, are described in the first part of this paper. An account is also given of the climate, soils, vegetation, population and rural structure, and public health of the island.The second part opens with a brief historical review of bilharziasis japonica in the Philippines up to 1953, when a control project was started. The objectives of this project, the areas selected, the census data and sampling used, and the techniques adopted are described. There follows a discussion of the prevalence of bilharziasis and its relationship to age, sex, occupation and environment; the prevalence of other common helminthic infections is also considered. In a section on the natural history and public health significance of bilharziasis, an approach to quantitative assessment of disease and disability and a method for evaluating the economic burden of bilharziasis are suggested. The incidence of disease in children of 5-9 years is reviewed in the same section. An analysis is made of possible strain differences of S. japonicum in Leyte, and the relative role of human and other animal hosts is assessed. The last section deals with the egg-laying habits of S. japonicum.