Ochlerotatus japonicus japonicus, a newly discovered nonindigenous mosquito species in NorthAmerica, and a colonized strain of Culex pipiens were compared for their vector competence for St. Louisencephalitis virus (SLE). Infection rates in Oc. j. japonicus were O-33% after feeding on chickens with viremiasbetween l0'and l0ar plaque-forming units (PFU)/ml of blood. In comparison, infection rates were 12-94%for Cx. pipiens that fed on the same chickens. When fed on chickens with viremias between 105.3 and 105.6 PFU/ml of blood, infection rates for Oc. j. japonicus and Cx. pipiens were similar, 96% and 100% respectively. After12-14 days of extrinsic incubation at 26"C, all 34 infected Oc. j. japonicus had a disseminated infection. Incontrast, only 23 (43%) of 54 infected Cx. pipiens had a disseminated infection after feeding on the samechickens. If they developed a disseminated infection, both species efficiently transmitted (>87%) SLE. Estimatedtransmission rates at viral doses sufficient to infect both of the tested species were 29-84% for Oc. j. japonicusand 3O-5O% for Cx. pipiens. Because of its continued geographic expansion, field and laboratory evidenceincriminating it as a vector of the closely related West Nile virus, and its ability to transmit SLE in the laboratory,Oc. j. japonicus should be considered as a potential enzootic or epizootic vector of SLE.