none Japan, Honshu, Aich. For the two parasitoid wasps, Ibalia leucospoides and Megarhyssa praecellens emerging together with their host woodwasp, Sirex nitobei, from trees of Pinus densiflora, the seasonal trends in number of emerging adults, their body size characteristics and fecundities were recorded to evaluate the life histories and parasitism patterns. The mean female body mass of M. praecellens was about 1.8 times (fall population) or about 4.1 times (spring population) larger than that of I. leucospoides. The difference in body size was remarkable also between sexes in both species. The spring population of M. praecellens was about 3.7 times (male) or 2.3 times (female) larger in mean body weight than the fall population. The mean egg length of M. praecellens was about 10 times larger than that of I. leucocpoides, whereas I. leucospoides produced about 50 times greater numbers of eggs than M. praecellens. The percentage of parasitism on S. nitobei by both parasitoids was considerably large, accounting for more than 60%. Our results suggested that two parasitoid species could utilize larvae of S. nitobei in wood as their hosts at different development stages of woodwasp in different manners: egg and/or 1st instar larvae parasitized by I. leucospoides and maturated larvae by M. praecellens.