The average total length lies between 23-25 cm. They may grow up to 30 cm and rarely more than 30 cm. Black Proteus can grow up to 40 cm or more. Males are somewhat smaller than females. Other sexually dimorphic characteristics include the shape and size of the cloaca during breeding activity, with the males having a larger and more elongated swollen cloaca than the females.
Sket and Arntzen (1994) described black populations of Proteus as a separates subspecies, and defended this taxonomic decision based on the limited amount of morphological (morphometric) differentiation that Arntzen and Sket (1997) observed between the two subspecies. However, Goricki and Tronteltj (2006) found little differentiation between the two subspecies at the molecular level and questioned whether the designation of subspecies was appropriate. Subsequently Tronteltj et al. (2007) reported that both "subspecies" were nested within a southeastern Slovenian clade of P. anguinus and that the division was in fact simply intra-lineage diversity.
Proteus is the only cave-adapted vertebrate in Europe. Current genetic research under the direction of Dr. Boris Sket of the University of Ljubljana suggests that Proteus anguinus is actually a complex of several species, with phylogenetic analysis revealing six cryptic lineages (see Trontelj et al. 2007).
Functional-morphological and environmental studies of Proteus have been performed at the Department of Biology, Biotechnical Faculty (BF), University of Ljubljana, Slovenia for more than thirty years, with the most recent twenty years under the guidance of Prof. dr. Boris Bulog.