Fatty acid (FA) compositions in leaves were investigated for two families of F(1) hybrids of Larix gmelinii var. japonica x L. kaempferi (F(1)) and their parent clones. Twenty-one FAs, from C(12) to C(32), were found in the leaves of both adult trees and seedlings. The levels of 18:1/(18:2 + 18:3) increased in the order L. kaempferi, F(1), and L. gmelinii var. japonica, with significant differences between L. gmelinii var. japonica and F(1) in adult trees, but these differences were not found in the seedlings. Moreover, in the adult trees, the 18:1/(18:2 + 18:3) levels in the neutral phospholipid fraction and the SigmaC(18)/SigmaC(16), especially in the glycolipid fraction, showed significant differences among the three species. These characteristics are discussed from the viewpoint of lipid synthesis in the endoplasmic reticulum and chloroplasts, and of the activities and substrate specificities in sequential FA desaturation. Linear discriminant analysis suggested that the FA compositions are useable as an index in the identification of hybrid seedlings.