WESTERN DIAMONDBACK RATTLESNAKE (Crotalus atrox)
Common.  Seen occasionally on the Main Loop Trail and more often on the Falls Trail especially near the Rio Grande.  Venomous.  Will rattle as warning. 

GOPHER SNAKE OR BULLSNAKE (Pituophus melanoleucus)
Common in most lower elevation areas of the park.  Can be seen on the Main Loop Trail or the Falls Trail.  Non-venomous.  Rodents are a major part of this snake's diet.

WESTERN TERRESTRIAL GARTER SNAKE (Thamnophus elegans)
Common especially along waterways.  Can be seen on the Main Loop Trail or anywhere in Frijoles Canyon.

STRIPED WHIPSNAKE (Masticophis taeniatus)
Common in Frijoles Canyon.  Longer and skinnier than Garter Snakes.  Have tendency to raise body off ground to scout for prey.