The closest relative of the axolotl is the tiger salamander; viable offspring can even be produced between the two species.
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: Josh More
