A Manitoba strain of Culex tarsalis and a Manitoba isolate of western encephalomyelitis virus were used to determine vector ability under laboratory conditions. The mosquitoes were infected by feeding upon viremic day-old chicks. Secondary hosts were chicks and mice, and they were easily infected by the bite of a single infected mosquito. Transmission rates generally increased over 31 days to 100% by the end of the experiment. Infection rates remained at or near that level throughout the experiment. The infection threshold level was low, log 2.3 TCID50. The combination of low infection threshold and high infection and transmission rates, indicates that Cx. tarsalis is highly susceptible to WE infection and can serve as an effective vector of WE in Manitoba.