planktivorous pelagic cyprinid of Lake Biwa Gnatopogon caeruleslens was transplanted to a dammed reservoir Lake Ono in Yamaguchi Prefecture in 1953, but nowadays fish seemingly intermediate between G. caerulescens and a brooklet-dwelling relative G. elongatus has propagated. Morphological and genetic comparisons clearly revealed the Lake Ono fish intermediacy between G. caerulescens and G. elongatus and strongly suggested that the Lake Ono opulation is a hybrid swarm between them. In order to transplant G. caerulescens to other waters from Lake Biwa hereafter, it should be introduced to sufficiently large and deep lakes capable of assuring its pelagic and planktivorous lifestyle, and contamination of G. elongatus should be eliminated at the same time.