The parasitoid Ichneumon eumerus is apparently host-specific to Maculinea rebeli , a rare and specialized butterfly. After a brief period of phytophagy, M. rebeli caterpillars live for 10 months in the brood chambers of Myrmica schencki ant nests, where they are protected by worker ants. The behaviour, morphology and allomones of I. eumerus enable it to oviposit in caterpillars inside ant nests. This study shows that it first locates M. schencki colonies by their odour, but neters only those nests that also contain Maculinea . It then selects the larger caterpillars for parasitism, which are usually the only ones that survive to pupate. Ichneumon eumerus emerges from the Maculinea pupa 11 months later. It is fiercely attacked by ants, but survives by being well-armoured and by spreading an allomone that provokes confusion and fighting among the ants. These findings are discussed in relation to the host-locating strategy of the parasitoid. It is suggested that this is the only possible way to parasitize "advanced" species of Maculinea , in which only a few but discernible groups of caterpillars are likely to survive. In contrast, "primitive" species of Maculinea , whose caterpillars survive less predictably at low densities in a large number of ant nests, are parasitized by ichneumonids that locate their hosts on the foodplant.