Vector competence of Aedes vexans (Meigen) and Culex pipiens pipiens L. (Diptera: Culicidae) for West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was compared. Infection rates of both species were similar 14 d after feeding on chickens, with WNV titers ranging from 10^4.2 to 10^8.7 plaque-forming units (PFU)/ml. Median infectious doses and 95% confidence intervals (CI) were 10^6.0(5.8,6.3) and 10^5.7(5.4,5.9) PFU for Ae. vexans and Cx. p. pipiens, respectively. WNV transmission was not observed in Ae. vexans that fed on chickens with WNV titers <10^5.0 PFU/ml, in contrast to a mean (95% CI) transmission rate of 7(2,18)% for Cx. p. pipiens. Mean WNV transmission rates for Ae. vexans and Cx. p. pipiens were 13(7,21)% and 10(5,19)%, respectively, after feeding on chickens with WNV titers of 10^{5.3 ± 0.1} and 10^{5.7 ± 0.1} PFU/ml, and 31(25,37)% and 41(30,53)% after feeding on chickens with WNV titers ≥10^{6.1 ± 0.1} PFU/ml. Time postinfection (p.i.) significantly influenced WNV transmission by Ae. vexans as indicated by a nearly 10-fold increase in transmission rate between days 7 and 14 p.i. Mean WNV load expectorated with saliva of Ae. vexans was 10^2.4(2.1,2.7) PFU, and it was not significantly affected by the titer of chickens on which they originally fed or time p.i. These data indicate that vector competence of the primarily mammalophilic Ae. vexans, which also feeds on birds, approaches that of Cx. p. pipiens for WNV. Because peridomestic mammals, such as cottontail rabbits, squirrels, and chipmunks, develop WNV titers infective for Ae. vexans, this species may play a significant role in WNV enzootic cycles.