This study was carried out to investigate some characteristics of selected trees recognized as natural hybrid between Pinus densiflora and Pinus thunbergii for improving resistance to pine leaf gall midge. `Che results obtained were as follows. l. In the mean annual increment and the growth in the period of latest 5 years of DBH, hybrid pines showed intermediate value of the parents, Pinus densiflora and Pa`nus thunbergii. However, there was remarkable variation among individuals of hybrid pine. 2. The trees having resin duct index(RDI) value larger than 0.1 for the position of resin duct in needle was classified as hybrid pine. The leaf and leaf sheath of Pinus thunbergii was longer than that of Pinus densiflora and the distance between rows of stomata on needles of Pinus thunbergii was longer than that of Pinus detasiflora, and the values of those observations on hybrid pine were intermediate between the parent species. However, differences among individual was observed. 3. In the size of cone and seed, and weight of 1000 seeds, like in the leaf characteristics, Pinus thunbergii showed higher value than Pa`nus densiflora and those values of hybrid were intermediate between the parent species. 4. Assuming isoenzyme ADH-B₂, ME-A₂, PGI-B₁, and PGI-B₂ alleles observed in the hybrids were introgressed from Pinus thunbergii, the hybrid pine can be easily identified by these isoenzyme alleles. However, the individuals which do not have those alleles can not be identified.