Active immunization of rainbow trout (Oncorhynchus mykiss), 26g and above against Enteric Redmouth disease (ERM) to reduce mortality caused by the Hagerman Type I (serotype 01) strain of Yersinia ruckeri. The vaccine is indicated for use in fish that have been vaccinated by immersion with AquaVacTM ERM within the previous 4 to 6 months.