We investigated the diet and foraging behavior of a putative generalist predator, the Terrestrial Gartersnake, (Thamnophis elegans) along a stream in central Washington State, USA. Snakes were collected, with the sex, mass, and snout-vent length (SVL) of each recorded. Snakes were categorized by SVL into 3 groups (<300, 301–500, and >500 mm SVL). Snake SVL and mass ranged from 190–723 mm SVL (x ¯  =  425, s  =  140.41), and 3.6–150.5 (x ¯  =  43.3, s  =  38.5) respectively. Of the 263 snakes collected, 138 contained 141 prey items. The most abundant prey items were crayfish (n =  79, 56.2% of total), followed by cottid (n =  32, 22.6%) and cyprinid (n =  17, 12.0%) fish, and slugs (n =  13, 9.2%). The smallest group of snakes fed primarily on cyprinid fish and some slugs. These snakes foraged (sit and wait within runs), while perched on rocks or logs. The intermediate size class foraged along riffles, runs, and in pools, and had the broadest diet, feeding on cottid and cyprinid fish, crayfish, and slugs. The largest snakes were the most specialized, feeding primarily on crayfish while foraging in pools. Our data show that some individuals in this population of T. elegans undergo an ontogenetic shift in both diet (vertebrate to invertebrate) and foraging behavior (sit and wait to underwater foraging). This is also the 1st population of Thamnophis documented to feed primarily on crayfish.