A series of in vivo comparisons between two marine vibrios pathogenic to pen-reared salmon demonstrated that each organism possessed protection-inducing, heat-stable components that were unique and others that were cross-protective. In all instances, however, immunization with the components of the homologous bacterium produced the highest degree of protection in juvenile coho salmon (Oncorhynchus kisutch) against subsequent live-bacteria challenge. Antigenic competition was not detected when fish were immunized with both bacteria, indicating that a bivalent vaccine would be effective against infection by either vibrio. Key words: fish disease, vibriosis, immunization, Oncorhynchus kisutch