An epizootic bacterial infection in rainbow trout (Oncorhynchus mykiss) had been occurred in Turkey during May-June 2001. The cumulative mortality was approximately 80 %. Infected trout had bilateral exophthalmia and congestion of blood-vessels in the pectoral and caudal fins. The body weights of trout were approximately 100 to 150 grams. Three bacterial isolates were obtained from diseased trout and biochemical, physiological and morphological characteristics were studied by conventional methods and Rapid ID 32 Strep System. All three isolates were found to have identical phenotypic characteristics and identified as Lactococcus garvieae.