Creatine kinase (CK) isoenzymes were examined as diagnostic indicators for pathophysiological changes in yellowtail infected with Lactococcus garvieae. Three isoenzymes of CK similar to those found in human were detected by agarose electrophoresis. CK isoenzymes in the brain, heart and intestine consisted of CK-MM, CK-MB and CK-BB. CK-BB was not detected in other tissues or plasma. Normal yellowtail plasma contained CK-MM and CK-MB, while CK-BB and CK-SP appeared in the plasma of infected fish. This phenomenon was found in both naturally and artificially infected fish. These results suggest that appearance of CK-BB and CK-SP in the plasma may reflect the pathophysiological changes due to the infection. Therefore, we consider that the examination of plasma CK isoenzymes is a pathophysiological diagnostic tool for the streptococcal infection in yellowtail.