The parasitid mite Peocilochirus necrophori Vitzthum (Acari: Parasitidae) is phoretic on several species of silphid beetles. However, individual mites have strong, genetically coded preferences for carrier species. P. necrophori is therefore either a single species that is genetically polymorphic for carrier preference, or else it is a sympatric sibling species complex. Regional variation in phoretic associations can be explained by differences in the relative proportions of the morphs.