Between February and April 1991, unusual numbers of bovine abortion around Antananarivo (central highlands, Madagascar) were reported by official veterinary services. Rift Valley fever (RVF) virus isolations were made from sixteen aborted foetuses and one dead calf in different foci. Using monoclonal antibodies, the isolated viruses were found to be different from the 1979 RVF strains isolated in Madagascar from mosquitoes and human laboratory infection, and closer to African RVF strains. In a bovine population--previously characterized by a negative or very low RVF antibody prevalence--a high prevalence of IgM antibodies (264/994: 26.5% positive) was revealed; the IgM prevalence in recently aborting females varied from 40 to 91%. Among 994 human sera tested by IgG-IFA (immunofluorescent antibody assay) and IgM ELISA, 8.2% and 4.5%, respectively, proved positive. A total of 11,371 mosquitoes (61% Culex antennatus) were collected in the epizootic areas and tested without any virus isolation. Extensive studies were conducted to determine the geographical extension and the impact of this epidemic on the highly susceptible livestock and human populations.