Cottus gratzianowi is distinguished from all congeners in Europe east of the Meuse (except C. koshewnikowi) in having the following combination of characters: no transverse dark bands on the pelvic fin; a single median chin canal pore; an incomplete lateral line not reaching behind the anal-fin insertion; and lateral line located considerably above the mid-line of the flank. It differs from C. koshewnikowi by a larger eye (horizontal diameter 23?28% HL, equal to or exceeding snout length vs. 16?25% HL, less than snout length), a rounded caudal fin (vs. commonly truncated), frequent presence of one to three branched rays in median part of the pectoral fin (vs. usual absence), an interrupted supratemporal canal commissure with 4 pores (vs. non-interrupted, with 3 pores), the pelvic fin extending to the anus in both sexes (vs. not reaching the anus), a short lateral-line canal extending to below the fifth to tenth ray of the second dorsal fin, with 15?24 pores, with few interruption (vs. commonly extending to below the 9th to 15th ray of the second dorsal fin, with modally 21?27 pores, with few to numerous interruptions), abdominal vertebrae commonly 10 (vs. 11), and contrasting black blotches on all fins including pelvic and anal fins (vs. no blotches on pelvic and anal fins) (Ref. 101366).