There is a generalized stimulus which attracts the males of Megarhyssa atrata (Fabricius), M. macrurus (Linnaeus) and M. greenei Viereck to form mixed aggregations and to engage in precopulatory behavior at the sites of emerging females of any of these species. In addition there is a species-specific stimulus which causes initiation of copulation by males of the same species as the emerging female. The generalized stimulus was identified as the chewing sound made by the female during excavation of her emergence burrow in the wood. The specific stimulus was not identified although sound within the range of 50-15,000 cps and vision were eliminated as possibilities. The females of these three species can locate accurately the host larvae in the wood and direct their ovipositors to them. The sense involved is either olfaction, detection of sound outside the range of 60-10,000 cps or some unknown sensory mechanism. Females oviposit only on hosts that are at a depth in the wood such that they can just be reached by the ovipositor when it is completely inserted.