It was found that yellowtails were readily infected with Pasteurella piscicida through the digestive tracts under an experimental condition. Healthy jevenile cultured yellowtails were used for experimentation. Newly isolated P.piscicida culture was stocked in a liquid nitrogen. container and was subcultivated just befboe use. A given quantity of the cells was put in a gelatin capsule together with some diets. The capsules were then administered directly to the stomack of the fish by means of a vinyle tube and piston rod. Groups of two or three fish were held in each 280 liter concrete tank with recircling and filtrating water system. All of the experimental fish were infected with the bacteria and died by the third to seventh day after administering the cells. Formation of white spots in the internal organs was observed in some victimes (Table 1 and 2). Recovery of P. piscicida from the blood of infected fish was investigated. Blood samples were serially taken from the fish by means of a syringe, introduced in Cuvier's tube, and living bacteria in the samples were counted by the plate method(Table 3). Recovery of P. piscicida from the digestive tracts of infected fish also was examined. The data obtained indicated that orally introduced bacteria were maintained in the intestine for long time(Table 4). Challenge of vaccinated yellowtail with a gastral administration of P. piscicida was tried. Experimental fish used were orally vaccinated with formalin-killed bacterin prepared from virulent P.piscicida broth-culture.The bacterin was given in a dose of 50 mg per fish per day for 21 days.The result suggested that oral vaccination with the killed bacterin was ineffective in the control of Pasteurelosis (Table 5).