Bunyaviruses including 53 strains of snowshoe hare (SSH) and 4 of Northway (NOR) were isolated from 132,428 unengorged adult female mosquitoes of 7 species collected throughout the boreal forest of the Yukon Territory and open woodland terrain in the Mackenzie Valley, Northwest Territories, Canada during 8 of 11 arctic summers from 1972 through 1982. Isolations of SSH virus were also achieved from mosquito larvae during 1974 and 1975. Percentage virus infection rates of important vectors were Aedes communis (0.038) and Culiseta inornata (0.124). Isolations of NOR virus were achieved during 1976 and 1978 only. Infection thresholds of SSH virus for Ae. communis were 0.1 mouse LD50, when virus transmission occurred both after virus feeding and after intrathoracic injection and Cs. inornata transmitted SSH virus after intrathoracic injection. Both Ae. communis and Cs. inornata were infected after injection of 3 plaque-forming units (PFU) NOR virus, and transmitted after injection of 300 PFU, but they also became infected after feeding on 30 PFU virus.