Experiments were designed to evaluate the vector competence of Culex tarsalis Coq. from an epidemic/epizootic area (Sioux City) in Iowa for a western equine encephalomyelitis virus (WEE-7738) and to elucidate maintenance mechanism of WEE in Iowa. WEE-7738 was isolated from Aedes trivittatus (Coq.) in 1977. Infection rate, dissemination rate, multiplication efficiency, and the transmission rate were examined for this virus in the SC strain of Cx. tarsalis and compared with previously published data. Susceptibility of the SC strain of Cx. tarsalis to WEE-7738 was similar to infection rates observed in studies conducted in California (Hardy et al. 1978; Reisen et al. 1996). However, the transmission of WEE-7738 by the SC strain of Cx. tarsalis was lower than transmission rates reported in other studies (Kramer et al. 1981; Reisen et al. 1993, 1996, 1997);Virus multiplication was more rapid in the first week after feeding than it was in the second week. The average amount of virus (titer) present in infected Cx. tarsalis was proportional to virus concentration in a blood-meal (P < 0.025);Mosquitoes exposed to 4.7--5.0 log TCID50/mosquito did not live as long as mosquitoes fed 2.7--3.0 log TCID50/mosquito or controls and Cx. tarsalis exposed to the lower virus titer did not live significantly longer than the controls;Infected Cx. tarsalis mosquitoes had 27.5% lower flight activity scores (P = 0.0035) and 26.1% fewer spontaneous flights (P = 0.024) than noninfected controls. Flight activity scores and initiation flights (flight numbers) decreased everyday. Virus infection did not affect on how long a mosquito flew in a 24-hr-period (the daily flying time) or the duration of individual flights. The circadian rhythm (spontaneous flight activity patterns) of infected mosquitoes were identical to those of controls;Vertical transmission (transovarial transmission) of WEE virus in Ae. trivittatus does not likely constitute maintenance mechanism of WEE virus in Iowa. The infection rate of Ae. trivittatus fed 7.0 log TCID50/ml of WEE-7738 was only 8.47%. None of F1 adult progeny was infected after eggs were maintained at 4°C for 4 months to simulate winter conditions.