In Kushima, Miyazaki Prefecture, Japan, we previously discovered three triploid hybrid seedlings of Miscanthus sacchariflorus. Therefore, at this location, we expected to discover some mature triploid Miscanthus plants. We collected rhizomes of 65 mature plants (31 M. sinensis; 34 M. sacchariflorus) from Kushima, and determined the 2C DNA contents of these plants by using propidium iodide flow cytometry. We compared these data with the 2C DNA contents of 170 plants from 4 other sites in Japan. We determined the average 2C DNA content of the 31 mature M. sinensis (5.22 pg) was markedly lower than that of the 34 mature M. sacchariflorus (8.72 pg). We found no mature plant showing intermediate DNA content between the 2 species. The 2C DNA content of M. sacchariflorus tended to be higher in Kushima than in any of the other 4 sites. Contrary to our prediction, we did not obtain triploid hybrids in mature Miscanthus plants at any of our study sites, including Kushima. In addition, we found a higher number of putative tetraploid hybrids occur in Kushima that in any of the other 4 sites. The large variations of relative DNA contents among M. sacchariflorus populations may be caused by natural hybridization.