This experiment had two objectives. One was to seek evidence for greater caution by California ground squirrels (Spermophilus beccheyi) when interacting with venomous rattlesnakes (Crotalus viridis), their most important snake predator, than when interacting with nonvenomous gopher snakes (Pituophis nielanoleucus), their second-most important snake predator. The second was to examine the role of olfaction in mediating snake-directed behavior by these squirrels, since many squirrel-snake encounters occur in burrows where visual cues are unavailable. In an illuminated room, we videotaped the responses by eight freely moving ground squirrels to the two snake species; the snakes were presented in large, transparent plastic bags that were either sealed (odor unavailable) or perforated (odor available). The results confirmed our predictions that the squirrels should behave more cautiously toward rattlesnakes than toward gopher snakes, and that olfaction is an important mediator of snake-directed behavior. Although the squirrels approached the two snake species equally often, they attended more to the rattlesnake than to the gopher snake, but remained farther away from the rattlesnake. When snake odor was available, the squirrels spent more time attending to the snakes, approached the snakes more frequently, approached the snakes in elongate postures more often and sand kicked more at the snakes. We proposed the following. i. The antipredator strategy of snake harassment by California ground squirrels is sensitive to risk, exhibiting attenuation where risk is higher. 2. Olfaction is likely to be even more important during squirrel-snake encounters in the dark, especially in burrows. 3. The ability to recognize snakes on the basis of odor may not depend on prior experience with snakes, and may circumvent difficulties associated with visual detection of snakes.