The interference of the floods in the planorbideos' life, whether contributing to the destruction of the focus in natural waters and irrigation systems, or in favouring their proliferation and dispersion of them, is mentioned. The effects of the flood and one of its consequences is the exacerbation of schistosomiasis in old focus and the appearing of new ones, are analysed. The occurency verified in the county of "São José dos Campos", State of São Paulo, Brazil, where the parasitosis increased in two old foci after the overflowing of the Paraiba river and its tributaries, is reported.
