Introduction: Yersinia ruckeri bacterin was the first commerciallyproduced fish vaccine and the formalin-killed whole-cell product continues to be highly effective. In the present study, we determined the influence of vaccination against yersiniosis on oxidative stress biomarkers and antioxidant defence in the muscle tissue of rainbow trout (Oncorhynchus mykiss) vaccinated against Y. ruckeri. Materials and Methods: The following parameters were evaluated in themuscles of rainbowtroutimmunized withY. ruckeri vaccine: the oxidative stress biomarkers 2-thiobarbituric acid reactive substances, aldehyde and ketone derivatives of oxidatively-modified proteins(OMB), as well as activities of antioxidant enzymes. Healthy fish were vaccinated orally with inactivated whole cells of a virulent strain of Y. ruckeri. One and 2 months after immunization the muscle samples were collected. Results: No significant difference was noted in lipid peroxidation level. Aldehyde and ketone derivatives of OMB in the vaccinated group were significantly lower in the second month compared with those in the first month after vaccination (P !0.05). The content of ketone derivatives of OMBinmuscles in thefirstmonthafter immunizationwas higher compared withuntreated controls. All of these culminated in adepletion of glutathione peroxidase activity and a low level of total antioxidant capacity. Conclusions: Immunization of fish with Yersinia vaccine is associated with induced free radical formation and oxidative stress. Free radicals would therefore be at least partially responsible for the induction of both humoral and cellular elements of immunity and increased protective immunity.