During and after the Rift Valley Fever epidemic in southern mauritania, we conducted epidemiological inquiries in order to determine epidemic and epizootic areas. The epidemic area was divided into two different zones. The first was located along the right bank of the Senegal River, around the town of Rosso. The second was located near the town of Kaedi. This epidemic area was included within the epizootic area, which stretched to the south, between Rosso and Saint-Louis. Several ecological changes had occurred in the recent past including erection of dams on the Senegal River for irrigation, and heavier rainfall than during previous years.