Three species, Megarhyssa atrata lineata, M. macrurus lunator, and M. greenei greenei were studied in beech-maple forests in Michigan. All are diurnal with similar activity patterns except that female M. atrata maintain a high degree of activity later in the day than do the others. Saturation deficits above 10 g/m3 inhibit activity whereas normal summer daytime temperatures do not. There does not seem to be a significant difference between time of emergence of females and of males in the spring, nor are there differences in seasonal occurrence of the various species except that M. greenei may persist slightly longer in the fall. Insects of both sexes and of all species tend to return repeatedly to the same log or stump with but a limited amount of movement between sites. Adults rest on the undersides of beech and maple leaves during inactive periods. Since there are more such places than individuals to use them, competition for shelter does not seem to occur. Neither do adults compete for food. The larvae of the three species are largely segregated ecologically in that they parasitize different segments of the host population. This depends on the facts that (1) the adults select a host larva at a depth in the wood so that complete insertion of the ovipositor at right angles to the surface of the wood will just reach it, (2) the ovipositors of the different species are of different lengths, and (3) the host larvae maintain themselves at about the same depth in the wood for most of their larval period. There is succession of species on a given log but not to the extent that the three species would be ecologically segregated without the above mechanism. The sequence of species in the succession seems to depend on differences in ovipositor length. Females are usually more abundant on logs when a species is invading a new log; males are usually more numerous in the declining phase. Adults can live at least 27 days. Predators in the area were robber flies and perhaps a chipmunk. Incidence of parasitism of Tremex by Megarhyssa was 26.7% in part of a log which was examined.