Crotalus helleri or Crotalus oreganus helleri, also known commonly as the Southern Pacific rattlesnake,[3] the black diamond rattlesnake,[4] and by several other common names, is a pit viper species[5] or subspecies[3] found in southwestern California and south into Baja California, Mexico, that is known for its regional variety of dangerous venom types.

[3] The specific or subspecific name, helleri, is in honor of American zoologist Edmund Heller.

[10] 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.

[11][12] 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,[13] than the venoms of other rattlesnakes, including the venom of C. helleri specimens of different provenance.

[10] 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. helleri, and that cross-breeding with the Mojave rattlesnake, which is geographically separated from neurotoxic Crotalus populations, was unlikely.