A Saskatchewan strain of the mosquito Culex tarsalis, transmitted a local strain of western equine encephalitis virus from chick to chick, between four and 44 days after an infective blood meal. At incubation temperatures of 69 and 75 degrees F, 120 transmissions occurred out of a possible 141, and all but seven of these were by single infected mosquitoes. At 75 degrees F virus titers in individual mosquitoes were more uniform and transmission was more efficient, than at 69 degrees F, although infection rates were similar at both temperatures. The minimum concentration of virus required to infect 50% of C.tarsalis was 10(2.5) intracerebral three-week old mouse LD(50) per 0.03 ml of donor blood. These findings provide direct evidence that C. tarsalis of Saskatchewan is a highly efficient vector of western equine encephalitis virus.