Ephemeral fever occurs in Kenya in epidemics which have been associated with periods of unusually heavy and prolonged rainfall. The epidemics persist for 2-3 years and involve most of the country. They occur in the same years as epizootics of Rift Valley fever, which is a mosquito-borne Phlebovirus infection. Sporadic outbreaks of the disease have been associated with local heavy rains; though one outbreak occurred during a dry period. Culicoides spp. were generally present in large numbers at the site of the outbreaks. Seroconversions were detected in sentinel cattle, when there had been no apparent clinical disease.