Aggregation of adult males is a widespread phenomenon among many species of Diptera, as well as among several groups of other insects (Downes 1969; McAlpine and Munroe 1968; Sullivan 1981). The function of such behavior, once controversial, is now generally conceded to be a mechanism for assembling the sexes in a sufficiently small area to ensure mating. The apparent scarcity of females, earlier thought to be an argument against this hypothesis, has been explained by the brevity of their visit, presumably only once in the lifetime of many, and only long enough to be intercepted by a waiting male (Papavero 1977). Adult acrocerids are rarely collected and little is known about their biology. Those species that have been reared are, during their larval stages, all internal parasitoids of true spiders (Araneae) (Schlinger 1981). The present paper describes the aggregation of males of Turbopsebius sulphuripes (Loews), and the presence of two others, Acrocera melanderi Cole and Pterodontia flavipes Gray, on a hilltop in southwestern Quebec.