An attempt was made on oral immunization of Anguilla japonica against Vibrio anguilllarum. Four groups of eels, 5g50g in body weight, were kept in separate experimental tanks. The first group was given the merzanin-killed organisms mixed with a formula powder food for about four months. The second group received the viable organisms in a similar way. The other two groups served as controls, being fed only food. Among those four groups, only the second group did produce circulating agglutinating antibody after three month's feeding, and a slight. protective immunity was demonstrated in the same group after four month's feeding. These results suggest that oral immunization deserves further work from a practical point of view.