The longevity of an Iowa strain of Culex tarsalis Coquillett fed blood meals containing 2 concentrations of western equine encephalomyelitis virus from Iowa (WEE-7738) was compared with that of Cx. tarsalis fed blood without virus. Females exposed to 4.7–5.0 log TCID₅₀ per mosquito of WEE-7738 did not live as long as mosquitoes exposed to 2.7–3.0 log TCID₅₀ per mosquito or controls. Only 1% of mosquitoes fed blood containing the higher virus concentration survived to day 18 after exposure. However, 13% of mosquitoes fed blood with the lower virus titer and 19.5% of the controls were still alive on day 18 after exposure. Flight activity scores of Cx. tarsalis infected with 4.7–5.0 log TCID₅₀ per mosquito of WEE-7738 were 27.5% lower, and there were 26.1% fewer spontaneous flights than noninfected controls from days 6–11 after infection. After day 8 after infection, infected Cx. tarsalis had 37.1% lower activity scores and 40.0% fewer spontaneous flights than noninfected controls. Virus infection did not affect how long a mosquito flew in a 24-h period (the daily flying time) or the duration of individual flights. The spontaneous flight activity pattern (circadian rhythm) of infected mosquitoes was identical to those of controls. Both infected and noninfected mosquitoes began spontaneous flight activity at 2000–2100 hours (CST) and were active throughout the entire dark phase of the 24-h cycle. Although mosquitoes were active throughout the night, there was a burst or peak of activity between 2200 and 2300 hours when the complete dark cycle began. These results indicate that the adverse effect of WEE infection on longevity and spontaneous flight activity of Cx. tarsalis may decrease vectorial capacity of Cx. tarsalis for WEE.