The susceptibility of Aedes triseriatus (Say) (Diptera: Culicidae) to low levels of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) was determined and compared with that of Culex pipiens L. to assess the likelihood of its participation in an enzootic cycle involving mammals. Ae. triseriatus and Cx. pipiens were exposed to WNV by feeding on baby chickens with WNV serum titers ranging from 10^{4.1 ± 0.1} to 10^{8.6 ± 0.1} plaque-forming units (PFU)/ml and from 10^{4.1 ± 0.1} to 10^7.0 PFU/ml, respectively. Infection rates and 95% confidence intervals (CIs) of 8% (4, 14) and 25% (15, 38) occurred in Ae. triseriatus and Cx. pipiens after feeding on chickens with WNV titers of 10^{4.1 ± 0.1} PFU/ml and increased to 65% (49, 79) and 100% (72, 100) in Ae. triseriatus and Cx. pipiens after feeding on chickens with titers of 10^{7.1 ± 0.1} PFU/ml. The mean infection rate of Ae. triseriatus ranged from 97% (84, 100) to 100% (79, 100) after feeding on chickens with WNV titers of ≥10^8.2 PFU/ml. The infectious dose (ID)₅₀ values for Ae. triseriatus and Cx. pipiens were 10^{6.5 (6.4, 6.7)} and 10^{4.9 (4.6, 5.1)} PFU/ml, respectively. The combined estimated transmission rate of Ae. triseriatus at 14 and 18 d after feeding on chickens with a mean WNV titer of 10^{8.6 ± 0.1} PFU/ml was 55%. Although Ae. triseriatus is significantly less susceptible to WNV than Cx. pipiens, the susceptibility of Ae. triseriatus to WNV titers <10^5.0 PFU/ml and its ability to transmit WNV suggest that Ae. triseriatus has the potential to be an enzootic vector among mammalian populations.