The purposes of this study were: (1) to describe the snake-directed antipredator behavior of rock squirrels; (2) to assess whether rock squirrels distinguish nonvenomous gopher snakes from venomous rattlesnakes; (3) to compare antisnake behavior in a snake-rare urban site and a snake-abundant wilderness site as a means of assessing whether natural selection or experience has generated population differences in behavior; (4) to assess snake densities in the two study sites; (5) to compare the antisnake behavior of rock squirrels with that of their closest relatives, California ground squirrels (Spermophilus beecheyi), a species that appears to differ from rock squirrels in exhibiting marked sexual-size dimorphism; and (6) to gather additional data on sexual size dimorphism in these two ground squirrel species. We tethered nonvenomous gopher snakes (Pituophis melanoleucus) and venomous western diamondback rattlesnakes (Crotalus atrox) in the field near burrows of marked squirrels and videotaped the ensuing interactions. Rock squirrels from both urban and wilderness populations confronted snakes while waving their fluffed tails from side to side, throwing substrate at the snakes, and even attacking snakes on occasion. Survey data confirmed large differences in snake densities between the two sites. Squirrels from the snake-abundant wilderness site distinguished rattlesnakes from gopher snakes, but squirrels from the snake-rare urban site did not. Since these squirrels show similar evidence of selection from snakes, as revealed by their equivalent physiological resistance to rattlesnake venom, we attributed these behavioral differences to the effects of snake experience. Rock squirrel antisnake behavior was very similar to that of California ground squirrels. Where the two species' behavior was dissimilar, the differences may be due in part to the interspecies variation in sexual size dimorphism confirmed in this study, and to the greater number of rattlesnake species that rock squirrels encounter.