If global warming occurs in California, daily mean temperatures may increase by 3 to 5°C, precipitation patterns will change, and sea level may rise 1 m. Studies were done on effect of temperature changes on survival of Culex tarsalis Coquillett, the primary vector of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses, in two regions where temperatures differed by 5°C. Daily mortality of adult vectors increased by 1% for each 1°C increase in temperature. At 25°C, only 5% of Cx. tarsalis survived for 8 or more days, the time required for extrinsic incubation of these viruses. Extrinsic incubation times for these viruses shortened when temperatures were increased from 18 to 25°C. WEE virus infection was modulated and transmission decreased at 32°C. If temperatures in the warmer region increase by 5°C, WEE virus may disappear and SLE virus would persist. In the cooler region, a 5°C increase would decrease vector survivorship and virus activity in midsummer. In North America, epidemics of WEE have prevailed above a 21°C isotherm and those of SLE below this isotherm. With global warming, epidemics of these viruses could extend into currently unreceptive northern areas. WEE virus would disappear from more southern regions. Geographic distribution of vector, human, and animal populations could be altered. North America could become more receptive to invasion by tropical vectors and diseases.