Proteus anguinus has a slender, elongate body averaging 23 to 25 cm in length from head to tail. The torso is cylindrical and has about two-dozen costal grooves along each side. The tail is laterally compressed, with small fins running along the top and bottom. The head is elongate with a broad snout that is blunted at the end. Its eyes are very small and do not even breach the skin. The skin is usually creamy white, but may have a pinkish hue. When exposed to light, the skin turns dark violet to black. Darkening of the skin is reversed when it is returned to the darkness. On the ventral side of the body the skin is translucent and reveals the contours of internal organs. Directly behind the head on each side are three large, feathery, bright pink gills. This species exhibits little sexual dimorphism in appearance, but females are larger than males.
Juveniles may be distinguished from adults by the faint yellow or red blotches on their skin, and their better developed eyes. Black olms, a variant of this species previously known as Proteus anguinus parkelj, have permanently dark skin and a shorter head.
Range length: 20 to 30 cm.
Average length: 23 to 25 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: female larger