Abstract The identification of interactions by predation in field can be considered as valuable complementary information for studies on feeding and interaction ecology. Here we report and describe two cases of opportunistic predation upon small mammals in forests of the Brazilian savanna (Cerrado): by the didelphid brown-eared woolly opossum Caluromys lanatus on the sigmodontine rodent Rhipidomys macrurus, and by the black-tufted-ear marmoset Callithrix penicillata on the didelphid Gracilianus agilis. Our observations suggest that opportunistic predation events on vertebrates can be advantageous for animals that feed mainly on plant items, as a way to obtain protein-rich resources.