Predator–prey interactions are complex relationships between both participants. Although many food items are available in the environment for a frog, it can choose to feed on one item instead of another, depending on several aspects of the prey, such as size, availability, abundance, mobility, and palatability. In this study we compared the frequency of each food item we observed in stomachs of the frog Chiasmocleis leucosticta with the availability of leaf litter arthropods we collected at the same place and time that of the frogs through Ivlev's electivity index. We verified that the electivity for ants and mites as food items in the diet of the population from Reserva Florestal de Morro Grande is an actual preference. Chiasmocleis leucosticta seems to elect smaller sized genera, such as Solenopsis and Pheidole, and avoid bigger and more aggressive ants, such as Pachycondyla, Camponotus, and Odontomachus. Also, we suggest that for C. leucosticta or even for other Microhylidae there is reason to believe that alkaloids are obtained as part of the diet; related studies on the biochemistry of this family, genus, and species are encouraged.