gudgeon, (species Gobio gobio), common small fish of the carp family, Cyprinidae, found in clear, fresh waters of Europe and northern Asia. A grayish or greenish fish, the gudgeon has a barbel at each corner of its mouth and a row of blackish spots along each side. Rarely exceeding a length of 20 cm (8 inches), it lives in schools and feeds mainly on small animals. It is edible and is used as fish bait.