We report on ecological interactions between the Gila monster (Heloderma suspectum) and the desert tortoise (Gopherus agassizii) in the Mojave Desert of southern Nevada. Eggs of desert tortoises are an important source of food for Gila monsters, and although tortoises are not always successful, they vigorously defend their nests from predation by Gila monsters. Shelters were co-occupied by these two species only during the nesting season for desert tortoises, and Gila monsters occupied shelters almost exclusively with female tortoises, probably because these shelters were nesting sites.