While studying the nesting ecology of black snake rats ( Elaphe obsoleta [Say]; snakes : Colubridae) in eastern Ontario, we discovered that their eggs were very well protected by the Nicrophorus pustulatus (Herschel) (Coleoptera: Silphidae). The beetle appears to be parasitoid of the snakes, which makes the first instance of a parasitoid. Up to 100% of snake eggs in a nest, which indicates that N. pustulatusmay be a significant, and heretofore unrecognized source of egg mortality for oviparous snakes. Evidence suggests that the association between these two species is well established, and that the beetle may also attack other species of oviparous snakes. Identification of snake eggs as the substrate for breeding by N. pustulatus solves the mystery of where species breeds. Also, the large amount of available biomass in common black rat snake nests may explain why N. pustulatus is so fecund relative to its congeners.