The first record of caudal fin-deformed wild fish, Conger conger (Linnaeus, 1758) from the northern Aegean Sea, Turkey was reported. The tail-deformed specimen with 370 mm Total length, 24 mm preorbital length, 14 mm eye diameter and 89 mm head length. The caudal region of the deformed specimen was completely deformed and fused, and the tail length of the abnormal fish was 33.3% when compared to the tail of the normal specimen. Possible reasons that are the cause of found abnormality such as viral, bacterial or environmental pollution are discussed. The abnormality could be considered an important indicator of environmentally induced stress since in the described area there is a significant pollution of heavy metals. The cause could be a spontaneous genetic mutation since the finding was a single specimen and not a frequent state in other fish within the studied area. Nevertheless, the present finding highlights the need for closer monitoring of the marine environment and for the identification of the specific factor that caused this abnormality.