Three consecutive year classes of juvenile chum salmon Oncorhynchus keta were vaccinated with Vibrio spp. bacterin, marked, and released into the marine environment. The return rates of adults from these releases were similar to those of unvaccinated control groups for two of the three year classes and were significantly lower for one of the year classes. There was no difference in size or age distribution at return between the vaccinated and control groups; thus, vaccination procedures did not increase survival or growth of these freely migrating chum salmon.