none. In southern Quebec, Megarhyssa atrata flies from May to September with a marked peak during the first 2 wk of June; males appear as soon as 19 May, and females are found as late as 9 September. During the summer 1992, M. atrata represented 50% of the population composed of three sympatric species of Megarhyssa sharing a common host, Tremex columba, developing in the American elm (Ulmus americana). However, M. atrata represented 60% of the total female population of the three species. The rate of parasitism estimated on the 107 host individuals examined was 57.9%. M. atrata develops as an ectoparasitoid and his life cycle, that probably lasts 2 yr, has four larval stages. M. atrata acts as a regulatory factor for Tremex columba populations, and could be used as a biological control agent in forestry. These data on the ecobiology of M. atrata are the first to be reported for Canada.