Sympetmm frequens is a very popular dragonfly in Japan. However, its biology is not well known. During its adult life to November a considerable migration from lowlands to highlands and back is observed, but whether they return to the original breeding site is not known. The vicariant species in Eurasia is S. depressiusculum which, however, has not been recorded from the greater part of China. No remarkable migratory flight is known for this species. On the Japanese islands, even as far south as the South Ryukyus, S. depressiusculum has sporadically been encountered, usually in October; the seasonal Mongolian high pressure is probably responsible for this overseas translocation. S. depressiusculum is found throughout the Korean Peninsula, but in the southern part of the Peninsula the body-size becomes almost as large as that of the Japanese S. frequens. Progeny reared from eggs of a female S. depressiusculum captured in Japan possessed bodily characters that suggested that they were hybrids between the two species. On some high mountains of Honshu (mainland of Japan) a number of small-sized, dark-coloured S. frequens were found late in the season when the majority of S. frequens had descended to the lower plain area. This might represent an isolated highland colony of this species. An hypothesis for the distribution and evolution of the two Sympetrum species is given.