Morphological, cytological and field observations on 20 populations of theAgrostis mertensii-flaccida complex were reported. The plants were found to be diploid, tetraploid, hexaploid, septaploid or octoploid with a basic chromosome number of x=7. The diploids were most wide-spread and diverse in habitat and morphology. The tetraploids were discovered in 3 sites in Hokkaido. The hexaploids and octoploids occurred on various mountains in central and northern Honshu as well as in some of Hokkaido. A septaploid plant was found on Mts. Daisetsu. Pollen studies showed that diploid and tetraploid plants had smaller grains when compared with those of the hexaploids and the octoploids. The diploids, though variable in morphology and ecology, seemed to belong to a taxon which was more or less deviated morphologically from the diploid or tetraploid taxa growing outside Japan. The tetraploids were indistinguishable morphologically from the diploids. The hexaploids in Honshu, which were clearly different from the hexaploids in Hokkaido in morphology, possessed unique spikelet features. The majority of the octoploids were found to be distinct from the diploids and the tetraploids in the length ratio of anther/lemma. Some morphological “marker” features which distinguished between the diploids and the octoploids were observed for all of their sympatric populations, though such “marker” traits varied according to the localities. The taxonomic and evolutionary relationships between these Japanese entities and the taxa growing outside Japan were discussed. In conclusion, setting aside some plants which require further investigations, the Japanese plants concerned were arranged into 3 species:Agrostis flaccida Hack., including 2X and 4X together with 3X previously reported;A. tateyamensis Tateoka, sp. nov., implying 6X in Honshu;A. mertensii Trin. including 8X. A description ofA. tateyamensis was given.