Coprolactistus whitakeri Radovsky & Krantz, a new genus and species of Macronyssidae, is described in all stases. Unlike other macronyssids, all of which are vertebrate parasites, C. whitakeri is predaceous, feeding on other mites in bat guano. Its morphology confirms the family assignment, and it has the peculiar macronyssid life cycle in which a regressive, nonfeeding deutonymph follows a normal, feeding protonymph. The chelicerae are typically macronyssid despite the predatory diet. Available information indicates that C. whitakeri evolved from macronyssid parasites. C. whitakeri is a distinctive species that is not obviously close to any other described macronyssid. It occurs in the guano of the big brown bat, Eptesicus fuscus, in Indiana and in the guano of an unidentified bat in Brazil.