Extensive water development has taken place in the north of Senegal over the last decade, resulting in a large increase in the amount of fresh water for irrigation. The objectives of the present study were to determine the prevalence and intensity of Schistosoma mansoni and S. haematobium in the Senegal river basin (SRB), and to ascertain the distribution of the snail species acting as intermediate hosts for both species of schistosomes. The schistosomiasis survey started in January 1994 and was completed in March 1995. Compared to studies before the construction of the Diama dam, there was a significant increase in both the prevalence and intensity of urinary and intestinal schistosomiasis in the human population in parts of the SRB. From the 9014 people who were registered from 180 villages and 4 towns (10 districts), 7750 were examined. S. mansoni was found in the lower valley (lower delta-Senegal river, lower delta-Lampsar river, upper delta, and diéré) but not in the middle valley. The mean prevalence ranged from 4.4% in the lower delta-Senegal River to 71.8% in the zone of Lac de Guiers, where prevalence and intensity of infection were higher on the eastern side of the lake (81.3% with a mean number of 2088 eggs/g of faeces) compared with the western side (50.3% with a mean 1111 eggs/g). S. haematobium was recorded throughout the area of study, ranging from a mean prevalence of 0.37% in diére (lower valley) to 41.5% in the lower valley (Lampsar river), where the mean egg count was 313/10 mL of urine. Physical and chemical changes to the environment have favoured the spread and increase in the populations of freshwater snails. The only snail involved in the transmission of S. mansoni was Biomphalaria pfeifferi. Five species of bulinid snails were present—Bulinus globosus, Bu. umbilicatus, Bu. senegalensis, Bu. forskalii and Bu. truncatus—but only the first 3 species were involved in the transmission of S. haematobium in the lower and middle valleys.