The aim of this study is the quantitative assessment of temporal variations in the composition and structure of a nocturnal flying beetle community in northeastern North America. In southern Québec (Canada), we collected by blacklight trapping a total of 33,382 individuals representing 625 Coleoptera species or morphospecies belonging to 63 families from a grassland at Scotstown during six years (2007–2012). Carabidae and Staphylinidae were the most abundant families and had the highest species richness. This blacklight trapping throughout the April–September period over many years provides critical information for assessments of beetle diversity in southern Québec, based on 1) the large proportion of species collected in small numbers (1–2 adults), 2) nearly 60% of species flying for only 1–3 months over six years, and 3) the large proportion of dominant/subdominant species in spite of strong year-to-year fluctuations in the relative abundance of species. Consequently, light trapping investigations in the future should be extended across many years to improve the inventory of species in a region, aid in the evaluation of the rarity of some Nearctic species, and study long-term effects of certain environmental factors (e.g., anthropogenic disturbance, climatic changes). We also studied the seasonal flight pattern for the 57 most abundant species (at least 100 individuals) including two eudominant species, Bembidion versicolor (LeConte) (Carabidae) (7.5% of the total catches) and Ectopria nervosa (Melsheimer) (Psephenidae) (6.8%); the flight peak of many species was during mid-summer. We determined the sex ratio for 23 abundant species; 12 species presented a female-biased sex ratio.