The Gila monster is the largest lizard native to the United States. They are a stout lizard with an elongated body, short limbs and a thick tail. A Gila over 500 mm (20 in.) in total length can be considered quite large. The largest wild Gila monster on record was 570 mm (22.5 in.) long. Hatchling are approximately 170 mm (7 in.) in total length and have a mass of about 30 grams (1 oz.).
The skin of the Gila monster has a "beadwork" appearance made up of individual rounded, raised scales. Embedded within the scales covering the body are osteoderms or small boney plates. Although such protective "bony skin" appears to have been fairly common in dinosaurs, the Gila is one of the few living reptiles with such extensive use of this type of armor.
The dorsal coloration of the Gila monster is black contrasted with pink or orange. In the southern variation, the reticulate Gila monster, the light markings are broken up to form a reticulated pattern. In the northern variation, the banded Gila monster, the light markings generally form an unbroken band across the back.
There is extensive variation in the appearance of Gila monsters throughout their range. There is not a clear distinction between the two forms based on body pattern. There are individual animals well within the range of the banded Gila monster that have the broken up appearance of a reticulate Gila and visa versa.
Gila Monsters of both forms are similarly patterned when they hatch—they both have a distinctly banded appearance . As they mature, the differences in pattern develop. This is referred to as ontogenic change.