AbstractBacteria capable of inhibiting growth of a pathogenicVibriosp. were isolated from the gastrointestinal tract of cultured halibut larvae during the first feeding and weaning stages. No such bacteria were found among isolates from the surface of eggs or the gastrointestinal tract of yolk sac larvae. The fraction of pathogen‐inhibitors among the total number of isolates ranged between 0–100% (first feeding) and 0–66% (weaning). All pathogen‐inhibitors were Gram‐negative rods, and 95% were oxidase and catalase positive fermentative isolates, capable of producing acid aerobically from a varying range of carbohydrates. These isolates possessed the characteristics of theVibrio/Aeromonas‐group, but only 19% were sensitive to the vibriostatic agent 0/129. Isolates from eggs and yolk sac larvae were dominated by bacteria belonging to theCytophaga/Flexibacter/Flavobacterium‐ group. The high fraction of isolates with the ability to inhibit growth of the pathogenicVibriosp. among the total number of isolates indicates that pathogen inhibition may be an important mutualistic role of the intestinal flora of early life stag