The immature stages of the Neotropical mistletoe butterfly Archonias brassolis tereas (Godart) (Pieridae: Pierini) are described and illustrated for the first time from the Atlantic Forest of Southeast Brazil. Eggs are laid in clusters on leaves of the mistletoe Phoradendron sp. (Viscaceae). Larvae are gregarious and underwent five instars. Mature caterpillars present yellow aposematic color pattern; pupae are yellow with black dorsal projections. Morphology, host plant use and behavior of immature stages are similar to species in the Catasticta group. An additional species of mistletoe butterfly, Brangas sp. (Lycaenidae), with gregarious and aposematic yellow caterpillars was found in sympatry on the same leaves of the host plant. These findings suggest that both interspecific competition and larval mimicry are important traits in the ecology and evolution of mistletoe butterflies.