The relative abundance and seasonal distribution of adult mosquitoes in southern Quebec, Canada, were studied using CDC light traps baited with carbon dioxide. Samples were collected twice a week from late May to early October 1977. Over 63,000 mosquitoes from 26 species in 7 genera were collected. Mansonia perturbans, Aedes aurifer and Anopheles walkeri were the most abundant, comprising 60%, 31% and 4% of the total collection respectively.