Noted, for the first time, is the occurrence of aggregations of males of the screwworm fly, Cochliomyio hominivorax (Coquerel). The continuous presence of males in each of two sites in south Texas was independent of the presence of cattle or of female screwworm flies. Males captured in autumn 1975 proved to be wild; those captured in spring 1976 were released, sterile insects. The behavior of males, wild or sterile, included vigorous conspecific and interspecific interactions. There was evidence of territoriality and competition for favored perching sites. The observations are consistent with an hypothesis that the male screwworm aggregations were mating assemblies similar to those known among other cyclorrhaphan Diptera. The significance of male aggregation to sterile fly liberations is discussed.