Although many species of tumbling flower beetles (Coleoptera: Mordellidae) are common and abundant, little is known of their life histories. Larvae of most species seem to be phytophagous, and acts of predation are considered rare and accidental. In this study, we dissected host plants of the endophytic mordellid Mordellistena aethiops Smith and subjected plant and insect samples to stable isotope analysis to determine trophic position. Dissections indicated that M. aethiops is a predator of the gall former Antistrophus rufus Gillette (Hymenoptera: Cynipidae) and possibly its parasitoids. Ratios of stable isotopes of nitrogen confirmed that prey constitute a significant proportion of the diet of the mordellid larvae but also revealed that some insects species and/or life stages, particularly hymenopteran parasitoids, may not enrich nitrogen isotopes as predicted by stable isotope theory, which currently is based primarily on predators.