There is debate as to whether the axolotl should be classified as its own species or as a subspecies of the tiger salamander.
The closest relative of Ambystoma mexicanum is thought to be Ambystoma tigrinum, the tiger salamander. Indeed, the larvae of these species are visually very similar. Some even consider the axolotl to be a subspecies of the tiger salamander; viable offspring can be produced between the two species in the laboratory, though no hybrids have, as of yet, been discovered in the wild.
Sources: (Bagnara, Armstrong, & Malacinski, 1989; Majchrzak, 2004; Smith, 1969)
Image: Scazon
