SummaryTo account for the usual monomorphism of distasteful animals and of Müllerian mimics, it is commonly suggested that predators take disproportionately more of the rare forms of distasteful prey. Two experiments were carried out to test this hypothesis, each comprising a number of trials in some of which one of two colours of artificial food was the rarer and in others of which the other form was the rarer. In one experiment, using wild passerines, selection was not frequency-dependent. In the other, using domestic chicks, the rarer form was taken disproportionately less than the commoner, contrary to expectation. The conditions under which selection is likely to favour or disfavour the rarer forms in distasteful prey are discussed. Selection favouring the rarer form may account for those unusual cases in which distasteful animals are polymorphic.