The efficacy of oral bacterin to control vibriosis was determined using pond cultured ayu (Plecoglossus altivelis). A formalin-killed whole cell bacterin of Vibrio anguillarum was fed to 322, 000 ayu in seven ponds eight to fifteen times at doses ranging from 0.08 to 0.16g bacterin/kg fish/day. Fish were exposed to natural infections due to vibriosis in the farm ponds. Experimental challenge was performed with fish lots where natural infection did not occur. Natural infection occurred in three ponds of vaccinated fish and three control ponds where bacterin was not fed. A decrease in the number of dead fish and the number of days of medication using oxolinic acid was shown in all of the vaccinated lots as compared to the non-vaccinated lots. Efficacy of the bacterin was assessed as the differences in the mortality profile and the need for medication during two months. The results of the experimental challenge showed the efficacy for more than 59 days. Combined use of oxolinic acid and bacterin did not reduce the efficacy of the bacterin. The smallest amount of bacterin yielding protection was eight feedings of 0.1 g/kg fish on alternate days. The total amount of bacterin required to vaccinate 322, 000 fish was approximately 1.4 kg equivalent to 4.3 mg/fish. The vibrio bacterin delivered by the oral route was shown to be highly effective when given to large commercial ayu populations showing ease in application and reduced stress for the fish.