The Black Sea, an inland sea, has contributed about 70% to the total fish production of Turkey for many years. Because of its low salinity and appropriate water temperature, both freshwater and marine fish such as salmon, trout, and sea bass can be cultured in the Black Sea. The aim of this study was to identify the etiological agent of mortality that occurred in sea bass in the Black Sea during July 2002. Bacteria isolated from kidneys and livers of infected fish were identified as Aeromonas salmonicida achromogenes according to morphological and biochemical characteristics, API 20NE results, and an agglutination test for A. salmonicida. The diseased fish had petechial hemorrhages on the bottom of the fins and lesions on the lateral and ventral sides of the body. Cumulative mortality was approximately 20%. This is the first report of A. salmonicida achromogenes associated with sea bass in Turkey.