Fungal cultivars of fungus-growing ants (Attini, Formicidae) are carried by dispersing queens from parent to offspring nest. This vertical cultivar transmission between generations is thought to result in long-term ant-fungus coevolution and selection for beneficial cultivar traits that maximize harvests and thus colony productivity. In contrast to this traditional view of vertical cultivar transmission, frequent horizontal cultivar transmission between ant species is implicated by a phylogenetic analysis of 72 cultivars propagated by two fungus-growing ant species coexisting sympatrically in central Panama. The two ant species are specialized on the same group of closely related cultivars, but in six of 12 cultivar clades identifiable within this group, cultivars from both ant species were united in the same clade. Five of these 'mixed' clades were supported by bootstrap values of about 90% or higher. In one instance, colonies from the two ant species cultivated the same, genetically identical, cultivar clone. These phylogenetic patterns indicate that: (i) cultivar exchanges between the two ant species occur routinely throughout ecological time; and that (ii) coevolutionary processes between ants and their fungi are more diffuse than previously assumed. Because the two ant species are specialized on a narrow group of closely related cultivars that they regularly exchange among each other, but not with other sympatric ant species, cultivar exchanges are constrained, most likely, by ant preferences for their own cultivar group or by stringent selection against transitions of ant lineages to distantly related cultivars.