Studies were conducted to determine the effect of environmental temperature on the susceptibility of Aedes taeniorhynchus mosquitoes for Venezuelan equine encephalitis (VEE) and Rift Valley fever (RVF) viruses. Mosquitoes reared at low temperature (19°C) were significantly more susceptible to infection with RVF virus (infection rate = 70%, 158 of 226) than were those mosquitoes reared at 26°C (infection rate = 48%, 135 of 280), regardless of the temperature at which mosquitoes were held after exposure to virus (19 or 26°C). Likewise, for Ae. taeniorhynchus exposed to VEE virus, the infection rate in mosquitoes reared at low temperature (97%, 236 of 243) was significantly greater than was that for mosquitoes reared at 26°C (78%, 217 of 280), regardless of the temperature at which mosquitoes were held after exposure to virus (19 or 26°C). Conversely, in mosquitoes infected with either RVF or VEE virus, virus disseminated from the midgut to the hemocoel more rapidly in mosquitoes held at 26°C than in those held at 19°C, regardless of the rearing temperature. Thus, a combination of low larval-rearing temperature and warm adult-holding temperature resulted in the most efficient mosquito transmission of both viruses.