Canadian peatlands are subject to disturbance and destruction, and drainage for agriculture is responsible for 85% of this degradation. Few studies have explicitly addressed the effects of habitat degradation on arthropod diversity in Nearctic peatlands. Because higher Diptera (Brachycera) in peatlands are diverse, are an important component of food webs, and exhibit a wide range of ecological requirements, we examined species richness, abundance, and community composition of Brachycera across transects at 1, 6, and 11 m from a collector drainage ditch in Johnville Bog and Forest Park, Quebec. In total, 1453 Brachycera were collected, representing 24 families and 166 species. Species diversity (based on Simpson's diversity index) and rarefaction-estimated species richness were higher at 6 and 11 m than at 1 m from the ditch, probably because of the homogeneous moss cover and moister conditions at greater distance from the ditch. Species composition also differed between 1 m and other distances, based on cluster analysis, multiresponse permutation procedures analysis, and the presence of five predaceous species that were significant indicator species 1 m from the drainage ditch. Our results suggest that anthropogenic degradation of hydrological conditions may be responsible for the low species richness and high dominance of a few species currently seen at the ditch margin.