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Although observations suggest pairwise coevolution in speci¢c 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-speci¢c ant^plant mutualism. The plant character is the prostoma, which is a small unligni¢ed 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 di¡erent shape. By performing precise measurements on the prostomata of three related myrmecophytes, on their speci¢c 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 ¢lter. 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.