Abstract When fry of chum salmon (Oncorhynchus keta) started to feed, they were vaccinated with Vibrio anguillarum bacterin by hyperosmotic infiltration. Mortalities of vaccinated and unvaccinated fish were compared in ambient and controlled challenge experiments. In the controlled challenge experiment, cumulative mortalities after 10 days were 6% in the vaccinated group and 63% in the unvaccinated group; in the ambient challenge experiment the respective cumulative mortalities after 70 days were 6% and 96%. The differences were statistically significant (P < 0.005), indicating that survival was highly contingent on vaccination under the conditions observed.