Geographic range: Neotropical: Endemic to Mexico. Western México from southern Sonora southward alog the Coastal Plain, foothills and valleys of Sinaloa, Nayarit, Jalisco, Colima and northwestern Michoacán, including in the northern part of its range. This species appears to occur mostly below 600 m. In the south of its range from near sea level well up into the Sierra de Coalcomán, Michoacán at 2225 m and Cerro Barolosa at 2400 m.

Physical characteristics: One of the largest rattlesnakes; specimens exceeding 150 cm are not rare, and the maximum size reported is 204.5 m, but there are some long term captives that are even larger. Large specimens have a prominent vertebral ridge on the anterior part of the body, shaped by high neural spines.
The ground color is olive green or yellowish brown; usually the largest snakes have a more conspicuous greenish cast. Dark punctuations are absent from the ground color, juveniles are reddish brown. The dorsal pattern consists of 26-41 diamond shaped blotches; they usually have a paler border and do not coalesces with the lateral series of blotches. In juveniles and young adults the dorsal blotches are usually reddish brown with paler centers. With age these blotches fade somewhat.
The top of the head is pale, and indistinct dark pattern is present on young snakes, but this usually disappears in adults except for a few dark scales. A dark postocular stripe about 3 scales wide extends from behind and below the eye. The tail is gray with a series of 5 to 12 darker bands.

Food habits: Feeds mostly on mammals.

Reproduction: There are few reports on reproduction behavior in this species, this snake is viviparous and give birth to 24 to 35 young.

Behavior: This snake appears to be most common or at least most active during the summer rainy season, and most individual are located crossing roads, although this rattlesnake had also been found in the dry season in Sinaloa, but this is the period of least activity.

Habitat: Thorn forest, tropical deciduous forest and ecotonal belt between tropical deciduous forest and pine oak forest, maybe in fir forest.

text: © Alfonso Delgadillo