Butterflies in the genus Maculinea have an unusual life-cycle in which they have evolved to exploit both specific host plants and colonies of Myrmica ants. Their reliance on two hosts with only partially overlapping ranges means that Maculinea butterfly populations are naturally patchy, and this mosaic distribution provides ideal conditions for coevolu-tion between the butterflies and their hosts. Here we give a summary of our research on the coevolution between Maculinea alcon and its host ants, Myrmica rubra and Myrmica ruginodis on Danish heathlands. The ants and butterflies show a geographic mosaic in their interactions, with some populations specializing on one host ant species, some on the other, and some utilizing both. This is primarily driven by an evolutionary arms race between the butterflies and ants in which the butterflies are selected to mimic the cu-ticular hydrocarbons of the ants more closely, while the ants are selected to change their cuticular hydrocarbons to be more distinct from the butterflies. This, in combination with fluctuations in population sizes of the ants (probably because of the negative effects of parasitism by the butterflies), leads to an interacting system that is highly dynamic in both space and time.