FWS Focus



The San Francisco garter snake is considered to be one of the most beautiful snakes in North America. It has a greenish-blue or blue belly, red on the top of its head, a wide pale stripe going down its back, and red and black stripes on the sides. 

Endemic to California, the San Francisco garter snake is only found on the San Francisco Peninsula from the edge of the San Francisco/San Mateo County lines south to the northern portion of Santa Cruz County. 

The snake is threatened by:

  • Illegal collection;
  • Habitat loss, fragmentation and degradation from commercial and residential development, road construction, artificial water impoundments, and agricultural activities;
  • Changes to aquatic habitat, including saltwater intrusion and drought;
  • Reduction in prey availability;
  • Predators or invasive species invasive species
    An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

    Learn more about invasive species
    that eat the snakes (and their prey); these include species such as bullfrogs and largemouth bass.

The San Francisco garter snake was listed as an endangered species on March 11, 1967. 

Scientific Name

Thamnophis sirtalis tetrataenia
Common Name
San Francisco Garter Snake
San Francisco Gartersnake
FWS Category

Location in Taxonomic Tree

Identification Numbers



Characteristic category

Life Cycle

Life Span

The lifespan of the San Francisco gartersnake is unknown; however, one study found that only 2% of females born live to age 5. 


Mating occurs from February into May but is especially concentrated in the first few warm days of March. San Francisco garter snakes are ovoviviparous. (This means fertilized eggs develop inside the female, but the embryo gains no nutritional substances from the female). In July and August, females give live birth to young. Brood size is variable, ranging from six to 35 young. 

Characteristic category



The San Francisco garter snake’s historical range extended from the San Francisco-San Mateo County line south along the base of the Santa Cruz Mountains into northern Santa Cruz County.

Recent surveys suggest that there has been very little decrease in the overall extent of the San Francisco garter snake range compared to its historical distribution; however, there are fewer populations and overall numbers of snakes within that range, and some sites have been extirpated. Remaining populations have been fragmented due to urbanization. Today, the snake is found in isolated populations, mostly along the coast in San Mateo County. 

The San Francisco garter snake requires both aquatic and upland habitat. They are often found in or next to aquatic freshwater habitat, including ponds, creeks, marshes, canals and other water sources, which they use for foraging and basking during the day. They also use grassy areas near water sources to regulate their body temperature, find cover, forage, mate and hibernate. During colder months, the snake moves into underground rodent burrows or under rocks for shelter. 


Land on which the natural dominant plant forms are grasses and forbs.


The land near a shore.


A considerable inland body of standing water.

River or Stream

A natural body of running water.


Areas such as marshes or swamps that are covered often intermittently with shallow water or have soil saturated with moisture.

Characteristic category



The San Francisco garter snake primarily eats Pacific tree frogs (Pseudacris regilla) and California red-legged frogs (Rana draytonii). They also eat immature newts, toads, small fish, worms and even rodents. 

Characteristic category

Physical Characteristics

Size & Shape

The San Francisco garter snake reaches a maximum total length of 47 inches (120 centimeters) for females, although the average length is 39 inches (100 centimeters). Male garter snakes are smaller than females, reaching about 83% of female length and 55% of female weight.

Characteristic category

Similar Species

Similar Species


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