The West Nile virus, a flavivirus commonly found in Africa, can occasionally infect humans and certain domestic animals. The vector for virus transmission is a mosquito, with wild birds serving as reservoirs. On September 3, 2000, the Institute for Public Health Surveillance was notified of suspected cases of West Nile virus encephalitis in horses in Lansargues (Hérault department). On September 8, the National Reference Center for Arboviruses confirmed the diagnosis by detecting specific IgM in the blood and detecting West Nile virus via PCR in a horse’s brain. The area where the cases were reported is near the Camargue in the Rhône Delta, a marshy region where large numbers of birds migrate from other parts of the world. On September 19, 39 horses in the Hérault and Gard departments exhibited clinical symptoms, and the diagnosis has already been confirmed by the National Reference Center for 8 of them. The maximum distance between the cases is 15 kilometers. No human cases have been detected to date.