A careful anatomical revision of 10,616 preserved specimens of snails from 76 localities of the State of S. Paulo, Brazil, was made with a view better to determining the geographical distribution of Biomphalaria straminea in the Neotropical Region of Southern Brazil. The analysis has shown that previous determinations were correct. The study was then complemented with a survey of information from the literature about distribution of the species. The distribution pattern of the species has expanded greatly over the last few years, perhaps an account of the construction of new dams, and the navigation system in the upper Paraná Basin. Epidemiological data have shown that B. straminea is a good host to S. mansoni. Continuous schistosomiasis control must be exercised so as to prevent the further expansion of the disease.