Life histories, morphological characteristics and host species were investigated under room conditions in Atanycolus initiator and Spathius brevicaudis, two major species of solitary wasps which parasitize subcortical beetles infesting Japanese pine trees. The duration of successive stages from egg hatching to adult emergence was about 16 days for both species. Females of both species required more time than males for development. A. initiator was about eight times larger in body weight and had an ovipositor 3.7 times longer than that of S. brevicaudis. While both wasps were polyphagous and some host species were common to them, three was a distinct discrimination in host size selection between the two parasitoid species. none