The behavioural patterns of Megarhyssa macrurus lunator, M. atrata lineata, and M. greenei greenei, all parasitic on wood-boring larvae of Tremex columba, were directly observed in nature and are herein described with a presentation of their functional significance. All three species had practically identical patterns of behaviour except where differences in morphology imposed modifications of a mechanical nature. Exploratory behaviour involves chiefly the tips of the antennae; the sensory organs located there are probably used to detect hosts although accessory receptors on the tip of the males’ abdomen may also be involved in detection of females. Two stimuli seem to be involved in mate detection and selection; a generalized one which attracts males of all three species to the log and brings them to a state of sexual responsiveness, and a species-specific one which stimulates males of only the appropriate species to initiate copulation. The latter is liberated only after the female cuts through the surface of the log.