For a chemotaxonomical investigation of Plagiochila acanthophylla Gott.subsp.japonica (Lac.) Inoue,nine materials of its fo.japonica(3),fo.fragilis (5) and fo.robusta (1) collected in various places of west Japan were supplied ; they were extracted with methanol and then analyzed by means of gaschromatography and mass spectroscopy to determine the relative contents of four main sesquiterpenoids, β-pompene (syn. gymnomitrene),　maalioxide, bicyclohumulenone and cyclicokorenone, to the whole ones. With regard to the compositions of these sesquiterpenoids, sharp differences were detected among these formae. Forma japonica contained all of the four components, their sum total was 85-95%, and among the components maalioxide was present with the particularly predominant contents. Forma fragilis contained three components other than cyclocolorenone, viz. β-pompene, maalioxide and bicyclohumulenone, and their sum total was 55-60%, but this forma included varied materials from ones containing β-pompene and maalioxide in almost equal contents to ones containing β-pompene in much larger proportion than maalioxide. Forma robusta contained β-pompene and maalioxide in each ca. 10% and the remaining part was constituted of many others sesquiterpenoids, but the components predominant over both the components were absent.