Although observations suggest pairwise coevolution in specific ant–plant symbioses, coevolutionary processes have rarely been demonstrated. We report on, what is to the authors' knowledge, the strongest evidence yet for reciprocal adaptation of morphological characters in a species–specific ant–plant mutualism. The plant character is the prostoma, which is a small unlignified organ at the apex of the domatia in which symbiotic ants excavate an entrance hole. Each myrmecophyte in the genus Leonardoxa has evolved a prostoma with a different shape. By performing precise measurements on the prostomata of three related myrmecophytes, on their specific associated ants and on the entrance holes excavated by symbiotic ants at the prostomata, we showed that correspondence of the plant and ant traits forms a morphological and behavioural filter. We have strong evidence for coevolution between the dimensions and shape of the symbiotic ants and the prostoma in one of the three ant–Leonardoxa associations.