The spatial distribution of horse flies and deer flies (adults and larvae) was studied within a bog and a fen of the Trois-Rivieres area, Quebec (46o21'N-72o31'W). Forty species were identified from 3 malaise traps (21 Chrysops, 13 Hybomitra, 5 Tabanus and 1 Atylotus). Seven species were found only in the bog and 4 only in the fen. Of the total number of species, 11 were also found as larvae during a simultaneous sampling survey of the peat moss substrate of these bogs ( 2 Chrysops, 8 Hybomitra and 1 Atylotus). Mean larval density was evaluated at 1.49 + 1.62 larvae/0.1 m2, randomly distributed (Poisson distribution). The potential productivity was estimated at approximately 146,000 larvae/ha. There is a statistically significant difference in larval distribution between open sites and forested areas. Adult trapping results and larval sampling data are compared and discussed.