Allozyme allelic frequency fluctuations in a hybrid swarm population of Gnathopogon caerulescens and G. elongatus (Cyprinidae) in Lake Ono, Yamaguchi Prefecture, Japan, monitored for twenty years from 1988 through 2007, indicated a fluctuating hybrid swarm equilibrium. Although future difficulty in the elimination of introgressant alleles was indicated, the possibility of future extinctions of some alleles at particular loci through genetic drift was also suggested.