Survival and infectivity of Flexibacter columnaris was found to decline in the presence of other species of bacteria, such as Aeromonas hydrophila (a fish pathogen) or Citrobacter freundii (nonpathogenic to fish). In the in vitro experiment, among the selected combination ratios of the initial density of F. columnaris to A. hydrophila or C. freundii (1 : 1, 10 : 1, and 1 : 10), the survival of F. columnaris was markedly reduced at 1 : 10. In the in vivo experiment, F. columnaris failed to invade loach when the initial density of A. hydrophila or C.freundii was approximately 100 times as much as F. columnaris. In the presence of the competitive bacteria, the density of F. columnaris found in the mucus of fish's body surface did not change during the period of experiment. On the other hand, in the absence of competitive bacterium, the density of F. columnaris in the mucus increased gradually, reached a peak after 24-48 hours and showed visible signs of infection. In the waters sampled from four different aquaculture facilities F. columnaris did not invade loach when the density of F. columnaris added was 1/100 of the total bacteria found in the test water.