In the present study, the etiological agent that caused mortality in goldfish at an aquarium company located in Bursa in northwestern Turkey was identified. Diseased fish were lethargic, displaying a loss of appetite, ascites, and exophthalmia. Internal examination revealed that the kidney was surrounded by exudate and that the spleen and liver were pale. Microbiological examination showed that the kidney, spleen, and liver were infected by gram-negative, cytochrome oxidase, catalase, and H2S positive rod-shaped bacteria identified as Shewanella putrefaciens. Identification of the bacteria was confirmed by 16S rRNA gene sequencing. Histopathological examination of the kidney, spleen, and liver revealed necrosis in kidney tubules and the presence of hepatocytes and hemosiderin in melanomacrophages. According to the disk diffusion method, isolates were sensitive to enrofloxacin (5 mu g), florfenicol (30 mu g), gentamycin (120 mu g), and sulfamehoxazole+ trimethoprim (25 mu g).