In 1879, Müller proposed that two brightly coloured distasteful butterfly species (co-models) that share a single warning-colour pattern would benefit by spreading the selective burden of educating predators. The mutual benefit of sharing warning signals among distasteful species, so-called müllerian mimicry, is supported by comparative evidence, theoretical studies and laboratory simulations; however, to date, this key exemplar of adaptive evolution has not been experimentally tested in the field. To measure natural selection generated by müllerian mimicry, I exploited the unusual polymorphism of Heliconius cydno (Lepidoptera: Nymphalidae). Here I show increased survival of H. cydno morphs that match locally abundant monomorphic co-model species. This study demonstrates müllerian mimicry in the field. It also shows that müllerian mimicry with several co-models generates geographically divergent selection, which explains the existence of polymorphism in distasteful species with warning coloration.