Predators of unionoid mussels are generally identified as fishes or aquatic mammals. During a mark and recapture study of the critically endangered mussel Popenaias popeii, we discovered a nymph of the dragonfly Gomphus militaris eating the gills of a gravid mussel; larvae and gill material were found in the nymph's gut. Many (15.2%) of the other mussels captured during a quantitative survey exhibited damage consistent with that inflicted by this dragonfly. Few non-gravid mussels were damaged and gravid mussels exhibited substantially more damage in gills used for brooding larvae than in gills not typically used for brooding. This previously unreported parasitic relationship may reflect a unique cost associated with reproduction and should be considered in the development of conservation strategies for P. popeii. Our observations underscore the need for basic ecological data when monitoring endangered species.