Maculinea butterflies are parasites of Myrmica ant nests. The Alcon blue, Maculinea alcon, is unusual in that it parasitizes the nests of several Myrmica species, using M. rubra, M. ruginodis and M. scabrinodis as hosts in different parts of Europe. In Denmark it uses M. rubra and M. ruginodis, but never M. scabrinodis. Some populations use one of these species exclusively, despite the presence of the alternative host, while others use both hosts simultaneously. To examine the basis of this specificity, and local coadaptation between host and parasite, we offered freshly emerged caterpillars of M. alcon from three populations differing in their host use to laboratory nests of all three recorded host ant species collected from each of the M. alcon populations. We measured the attractiveness of the caterpillars to their host ants as the time taken for them to be adopted by each ant colony. Caterpillars from all populations took longer to be adopted to M. scabrinodis nests than to nests of the other two ant species. Adoption times to M. rubra and M. ruginodis colonies differed: caterpillars from each of the two populations that used a single host species were adopted most quickly by that species when local ant colonies were used. When ant colonies collected from the other two sites were used, this pattern broke down, and there was either no difference in adoption time, or M. rubra adopted caterpillars more quickly. Adoption of caterpillars from the population that used both M. rubra and M. ruginodis as hosts took an order of magnitude longer than caterpillars from populations using a single host species. (C) 2001 The Association for the Study of Animal Behaviour.