Strains of California encephalitis virus (snowshoe hare subtype) were isolated from 8 of 475 pools comprising 23747 unengorged female mosquitoes of five species collected at three of six locations throughout the Mackenzie Valley of the Northwest Territories, Canada, from latitudes 60 to 69° N between 10 and 24 July 1976. Minimum field infection rates included 1:2734 for Aedes communis, 1:256 to 1:3662 for A. hexodontus and 1:911 to 1:1611 for A. punctor. Northway virus was also isolated from 1 of 3662 A. hexodontus mosquitoes collected at Inuvik (69° N, 135° W). Transmission of CE virus by A. communis infected by feeding on virus in defibrinated blood and incubation at 0, 13 and 23 °C for 13–20 days clearly demonstrates the importance of this species as a natural vector, and transmission of CE virus by Culiseta inornata after incubation at 0 and 13 °C following intrathoracic injection strengthens evidence of its role as a natural vector. Immunofiuorescence was less reliable than imunoperoxidase for detection of CE viral antigen in mosquito salivary glands.