The present study was undertaken to examine the antibacterial abilities of intestinal bacteria isolated from juveniles and larvae of Japanese flounder Paralichthys olivaceus. Newly hatched larvae of flounder were held in a 100 L plastic circular tank and fed rotifiers, Artemia nauplii and commercial feeds, depending on the developmental stage of the fish. Genera Aeromonas, Moraxella and Vibrio were predominantly isolated from the intestinal tracts of Japanese flounder at larval and juvenile stages, whereas Aeromonas, Bacillus, coryneforms, Moraxella, Pseudomonas and Vibrio were detected at high densities in live diets and artificial feeds. Antibacterial bacteria accounted for 1.7-24.3% of the intestinal isolates against Lactococcus garvieae, Pasteurella piscicida, Vibrio anguillarum and V. vulnificus. In particular, as much as 53.3% of Vibrio spp. other than Vibrio-swarmer isolated from 197-day-old juveniles inhibited the growth of P. piscicida. These results suggest that intestinal bacteria having antibacterial activity play a role in the prevention of infectious diseases.

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