Crotalus horridus, the timber rattlesnake, is a species of venomous pitviper found in the eastern United States. This is the only rattlesnake species in most of the populous northeastern United States. No subspecies are currently recognized.
Adults usually grow to an average length of 91–152 cm (35.8-59.8 in). The maximum reported length is 189.2 cm (74.5 in)(Klauber, 1956). Holt (1924) mentions a large specimen caught in Montgomery County, Alabama, that had a total length of 159 cm (62.6 in) and weighed 2.5 kg (5.5 lb). Large specimens can reportedly weigh as much as 4.5 kg (9.9 lb). Most timber rattlesnakes found measure less than 100 cm (39 in) and weigh 580–900 g (20–32 oz).
The dorsal scales are keeled and arranged in 21-26 scale rows at mid-body (usually 25 in the south and 23 in the north). The ventral scales number 158-177 in males and 163-183 in females. Males have 20-30 subcaudal scales while females have 15-26. The rostral scale is normally a little higher than it is wide. In the internasal-prefrontal area there are 4-22 scales that include 2 large, triangular internasal scales that border the rostral, followed by 2 large, quadrangular prefrontal scales (anterior canthals) that may contact each other along the midline, or may be separated by many small scales. Between the supraocular and internasal, only a single canthal scale is present. There are 5-7 intersupraocular scales. The number of prefoveal scales varies between 2 and 8. Usually the first supralabial scale is in broad contact with the prenasal scale, although slightly to moderately separated along its posteroventral margin by the most anterior prefoveals.
Dorsally they have a pattern of dark brown or black crossbands on a yellowish brown or grayish background. The crossbands have irregular zig-zag edges, and may be V-shaped or M-shaped. Often a rust-colored vertebral stripe is present. Ventrally they are yellowish, uniform or marked with black. Melanism is common, and some individuals are very dark, almost solid black.
