There is great ecological interest in carrying out a new inventory of southern Québec simuliids, including an exhaustive sampling in two lake outlet streams of the Mauricie region, because data from Québec were last reported only prior to 1980. Frequent sampling is needed to determine how species distributions change in response to climatic and environmental changes. Twenty-two of the 65 Québec species were collected in the present study. One new species for Québec was collected (Prosimulium magnum complex), and new distribution areas were found for some species. Exhaustive sampling clearly showed time- and space-related distribution variations of the univoltine and polyvoltine species. Seasonal appearance of species was synchronized at the two sites studied, but generation number was smaller at Lac du Barrage than at Lac Souris. The species-colonizing process seems to be environmentally determined stream-width and substrate-type influenced, as Simulium annulus, S. jenningsi. S. pictipes, and S. innoxium, were found in high-current-velocity falls. Artificial site colonization by S. aureum in a peri-urban environment had not been observed among simuliids prior to this study, in contrast to many mosquito species.