Dietary studies are important for understanding predator–prey relationships and species interactions because they provide information on the trophic resources available to predators and their potential impact on prey populations. We relied on stomach contents of museum specimens and literature records to examine ontogenetic (size-related), sexual, seasonal, and geographic variation in the feeding habits of Sidewinders, Crotalus cerastes. Sidewinders fed primarily on lizards and slightly less frequently on mammals; birds and snakes were rarely consumed. The vast majority of C. cerastes consumed single prey items ingested head-first. Juvenile and adult female Sidewinders consumed lizards and mammals with similar frequency. We observed an ontogenetic shift in feeding patterns of adult male C. cerastes because they included more mammals in their diets, compared with juvenile males. Sidewinders are classic ambush (sit-and-wait) predators and, as predicted by theory, actively foraging lizards and mammals comprise a considerable fraction of their prey. We documented seasonal shifts in the feeding patterns of Sidewinders, with snakes consuming a greater proportion of lizards during early spring and autumn, and a greater percentage of mammals during late spring and summer. This dietary shift likely results from seasonal changes in the activity patterns of C. cerastes, because individuals can be diurnally active during early spring and autumn but are predominantly nocturnal during late spring and summer. Adult male and female Sidewinders from the Mojave and the Sonoran deserts consumed similar proportions of lizards and mammals. Our findings regarding the trophic habits of C. cerastes contribute to our understanding of the ecology of terrestrial, venomous predators.