Information is presented on the occurrence of spinal deformities in natural populations of grass goby, Zosterisessor ophiocephalus (Pallas, 1811) in the Karin Sea, Eastern Middle Adriatic (44°09’N-15°37’E). During 2002, the spinal deformity levels varied between 8.6% (in March) and 20.9% (in May), and 16.5% in total catch. The spinal column is deformed in one to few places from vertical to horizontal angles, and the deformities are visible on the fish body immediately after catching. The possible reasons for such deformities are discussed.