The black-tailed rattlesnake (Crotalus molossus) is a venomous pit viper species found in the southwestern United States and Mexico.

Alternate common names are green rattler,[5] and Northern black-tailed rattlesnake.

As their name implies, one of their most distinguishing features is, despite variations in body color, entirely black tail scales.

Often, this rattlesnake has a black band across its eyes extending diagonally down to the corners of its mouth, forming a sort of facial 'mask'.

Its distribution reaches a maximum elevation of 2930 m, although it has been recorded at as high as 6900 feet at the McDonald Observatory in the Davis Mountains of west Texas.

(Fort Webster, Santa Rita del Cobre, Grant County, New Mexico, USA).

They are variable in their form of locomotion depending on what substrate they need to traverse and will actively change between sidewinding or rectilinear movement.

Crotalus molossus is considered to be one of the most docile rattlesnakes because of its calm demeanor and curious nature.

The female gives birth to live young in the summer, and the babies stay with the mother only until they wander off on their own, usually in less than a day or two.