Crotalus oreganus is a venomous pitviper species found in North America in the western United States, parts of British Columbia and northwestern Mexico. Seven subspecies are currently recognized, including the nominate subspecies described here. The size of this species varies greatly, with some populations being stunted and others growing very large. Mainland specimens often reach 100 cm in length, with the largest on record being 162.6 cm for C. o. oreganus. This species, in its various forms, shows considerable ontogenetic variation. Juveniles usually have more or less distinct patterns, but this fades as the animals mature. The color of the iris often matches the ground color, which may be bronze, gold, different shades of tan, pink or gray.