Description: Adults are 24–55 inches in length. The color pattern consists of a pale brown, gray-brown, or yellowish brown ground color overlaid with a series of large, dark brown dorsal blotches that may or may not have pale centers. The blotches are more diamond shaped, as opposed to those of C. o. oreganus that are more hexagonal, and are bordered by light scales. The tail rings are not clearly defined. In juveniles, the end of the tail is bright orange, but this turns to brown as the snakes mature. In adults, the base of the tail and the first segment of the rattle are brown. The postocular stripe is moderately to very clearly defined. In juveniles, this stripe is bordered above by a pale stripe, but as the snakes mature this turns to drab yellow or brown. A conspicuous pale crossbar is sometimes present across the supraoculars, after which the head is a uniform dark color. In some older snakes the head is mostly dark with almost no trace of the supraorbital crossbar, or none at all.

Habitat: Found in a wide range of habitats from seaside dunes, to desert scrub, grassy plains, rocky hillsides, chaparral, open woodlands, and agricultural areas.

Range: This subspecies, Crotalus oreganus helleri - Southern Pacific Rattlesnake, is found in California from Santa Barbara County, where there is a wide zone of intergradation with the Northern Pacific Rattlesnake north to around Morro Bay, east to the central valley and the desert slopes of the transverse and peninsular ranges, and south into the middle of the Baja California peninsula. Ranges north of the transverse ranges into the Mojave Desert in the Antelope Valley and just south of Barstow. Also found on Santa Cruz and Santa Catalina Islands

Found in these States: CA

Venom: Some populations of Crotalus helleri have a neurotoxic venom that is very similar to the extremely dangerous Mojave rattlesnake (the "Mojave Green") toxin in the way it attacks the nervous system. Other populations can have hemotoxic and myotoxic venom that is more typical among rattlesnakes and though less dangerous, can also can give a fatal bite. Thus, depending on where the bite was sustained, envenomation from this snake can require a much higher dose of Crotalidae polyvalent immune fab ("Crofab"), an antivenom used to treat the bite of North American pit vipers, than the venoms of other rattlesnakes, including the venom of C.o. helleri specimens of different provenance. In a survey of various populations of Crotalus in California, every sampled specimen with disabling neurotoxic venom had originated near Idyllwild, California, in the San Jacinto Mountains. Scientists considered the intraspecific variety of venom types "medically significant", while hypothesizing that evolutionary pressures, driven by regional habitat differences and the associated challenges of hunting prey in each, could have been behind the variation of venom types in C.o. helleri, and that cross-breeding with the Mojave rattlesnake, which is geographically separated from neurotoxic Crotalus populations, was unlikely.

Diet: Using its heat-sensing facial pits to locate prey, C. oreganus eats birds, bird eggs, and small mammals, from mice to rabbits. It also eats small reptiles and amphibians. The juveniles eat insects.

Reproduction: Males reach sexual maturity in two to four years, with most reproducing for the first time in their third year. Females in contrast mature in three to seven years, with most first reproducing in their fourth year, however northern populations in British Columbia are known to first reproduce as late as their sixth, seventh, or eighth year. The reproductive cycle of females is normally biennial, although 10% may produce litters in two consecutive years, and one case of a three year interval has been reported. Crotalus oreganus typically mate in the spring after emerging from winter dormancy, although British Columbia populations have been reported to mate in the fall before dormancy. The gestation period was reported to be about 90 days in wild snakes from Idaho, but periods of 143 and 425 days have been reported for individuals in captivity. Females usually fast while they are gravid. They are viviparous, producing live young. Parturition of 1–15 (average 3–8) young usually occurs in August or September, with neonates ranging 19–28 cm. (average 25.2 cm.) in total length. One report of 25 young is regarded as questionable by some herpetologist. Mating usually takes place in concealed areas, like burrows, crevices of rocks and logs, or dense brush. Mating snakes are highly sensitive to disturbance and are quick to separate.

Status: Listed as Least Concern in view of its wide distribution, tolerance of a broad range of habitats, presumed large population, and because it is unlikely to be declining fast enough to qualify for listing in a more threatened category.

Taxonomy: