Experiments were designed to evaluate the vector competence of Culex tarsalis Coquillet from an area (Sioux City) where Cx. tarsalis is most abundant in Iowa for western equine encephalomyelitis virus (WEE-7738). WEE-7738 was isolated from Aedes trivittatus (Coquillet) collected in Ames, IA, in 1977. Infection rate, dissemination rate, multiplication efficiency, and transmission rate were determined for this virus in the SC strain of Cx. tarsalis. SC strain of Cx. tarsalis was as susceptible to WEE-7738 as Californian strains of Cx. tarsalis were to BFS1703 strain of WEE; OID₅₀ of SC Cx. tarsalis was 2.63 log TCID₅₀ per mosquito and OID₅₀ of Californian strains of Cx. tarsalis were 2.0–4.1 log PFU per mosquito. However, transmission of WEE-7738 (4.2%) by the SC strain of Cx. tarsalis was lower than those (10–60%) reported in other studies.