The only venomous snakes in California are the rattlesnakes and there are eight species:

(Pictures provided by California Reptiles and Amphibians at www.californiaherps.com)
 
 
The following five pictures thanks to Biology of the Rattlesnake Symposium at Loma Linda University at www.roblee.com/rattlesnakes
 
The Western Diamondback (Crotalus atrox)

The Western Diamondback (Crotalus atrox)

The Red Diamondback (Crotalus ruber)

The Red Diamondback (Crotalus ruber)

The Speckled Rattlesnake (Crotalus mitchellii)

The Speckled Rattlesnake (Crotalus mitchellii)

The Mojave Rattlesnake (Crotalus scutalatus), sometimes called "the Mojave Green Rattlesnake")

The Mojave Rattlesnake (Crotalus scutalatus), sometimes called “the Mojave Green Rattlesnake”)

 
The physical appearance of each snake species is variable and it is difficult to tell what species of snake one is looking at. Some general principles in distinguishing poisonous snakes are:
 

  • Broad, triangular head with a noticeable “neck” behind the head.
  • Vertical pupils (non-poisonous snakes have round pupils) though hopefully one would not be close enough to evaluate this.
  • The Crotalines are also called “pit vipers” because they have heat-sensing “pits” on t heir faces between the eye and nostril. The pits help them locate prey.

 
Rattlesnakes can be found in rural areas as well as suburban areas where there is sufficient natural habitat. In Northern California snakes will hibernate during cold months and are active March through September. In Southern California they are active all year round.