Rift Valley fever (RVF) is an expanding zoonotic disease transmitted from ruminant to ruminant by Culicidae mosquitoes. In 2004, a longitudinal serological survey was performed on small ruminants in the Ferlo are (Senegal) to study RVF transmission and compared the results with those obtained from the same study in 2003. The results confirm that the disease is endemic and that the spatial transmission of RVF is highly heterogeneous. The virus could be maintained during dry season by transovarian transmission in Aedes vexans. Further studies are needed to improve the understanding of the epidemiological cycle of RVF in this region to implement adapted surveillance measures.