During a five-year field study, we made observations and conducted experiments to demonstrate unequivocally that Euphyonarthex phyllostoma (Fulgoromorpha: Tettigometridae) is a myrmecophile. Isolated adults and colonies always were found in association with ants. Colonies were associated only with Camponotus brutus or C. acvapimensis (Formicinae), whereas isolated adults were attended by ants belonging to several species of Formicinae, Dolichoderinae, and Myrmicinae. The size of the planthopper colonies reached higher levels when attended by C. brutus than by C. acvapimensis. Experiments using ant exclusion showed that both ant species protected egg masses against parasitic wasps, but egg masses were less parasitized on trees occupied by C. brutus than on those occupied by C. acvapimensis (P = 0.0052). The production of egg masses by female hoppers was recorded only when C. brutus, C. acvapimensis, or the myrmicine ant Myrmicaria opaciventris attended the hopper. In both former cases, the presence of ants influenced the aggregation of the nymphs as they dispersed when ants were excluded. The aggregation of the nymphs ensured that they were properly attended. Parental care by the females was reduced to their presence above or close to the egg masses. In fact, specialized workers of the attending ant species protected the egg masses as well as nymphs.