Serological and virus isolation studies were conducted in the Trois-Rivieres area, Province of Quebec, from 1974 to 1978. Antibodies to St. Louis encephalitis, Powassan, western equine encephalitis and California serogroup viruses were detected in human and animal sera. Seven strains of snowshoe hare virus were isolated, including four from Aedes punctor and one each from Ae. aurifer, Ae. communis and Ae. excrucians. This is the first reported isolation of snowshoe hare virus from Ae. aurifer mosquitoes.