The results of the studies about foci of Schistosomiasis in the localities of Pedro de Toledo and Itariri, are presented with special reference to the natural infection rate, reached for the Biomphalaria tenagophila species. This snails which transmits Schistosomiasis in "Vale do Ribeira", was also found in the following counties in that region of the State of São Paulo: Eldorado, Iguape, Itariri, Jacupiranga, Juquiá, Miracatu, Pariqueraçu, Pedro de Toledo, Registro and Sete Barras. The natural infection rates of 68.1%; 75.0%; and 81.3% obtained in some samples of B. tenagophila, the highest till now verified of this species, prove once more the great susceptibility to the infection by Schistosoma mansoni.
