During the summer of 2006, an outbreak of lactococcosis took place in a rainbow trout farm in the southern Aegean region of Turkey. Clinically lethargie, erratic swimming, darkness in colour and exophthalmia were observed. Histopathological examination revealed epicarditis, peritonitis, enteritis and panophthalmitis. The gills had lamellar edema, hemorrhage and telangiectasia. Histopathological findings were also found in the liver, kidney, pyloric caeca and swim-bladder. In conclusion, the bacterium isolated from the rainbow trout farm was identified as Lactococcus garvieae by biochemically, and its pathological affects on the rainbow trout tissues were widely evaluated firstly in Turkey.