Intestinal schistosomiasis has never been reported in the delta of the Senegal River Basin. A year and a half after the Diama dam became operational, a first case of Schistosoma mansoni infection was detected at Richard-Toll. In 1988 and 1989, 1,935 stool examinations out of a sample of 3,926 were found positive for S. mansoni. The progression of positive stool samples is rapid in all neighbourhoods of Richard-Toll: 1.9% positive examinations in 1988 compared to 71.5% in the last quarter of 1989. One thousand nine hundred and sixty nine Biomphalaria pfeifferi were collected during several malacological surveys carried out from February 1989 to December 1989 at three observation points. An emission of S. mansoni furcocercariae was determined in 44 out of 926 B. pfeifferi (4.75%) examined during this same period. Intestinal schistosomiasis is new in this area and spreading quickly. Rapid intervention measures are required immediately.