A dietary exposure bioassay with larvae of the ground beetle, Poecilus chalcites (Say) (Coleoptera: Carabidae), was developed to assess potential nontarget impacts of corn event MON 863 expressing the Cry3Bb1 protein for control of corn rootworms, Diabrotica spp. (Coleoptera: Chrysomelidae). The assay involved 28 d of continuous exposure of P. chalcites larvae to an artificial diet treated with a maximum hazard dose (930 μg/g of diet) of the Cry3Bb1 protein. Results from this study showed that the Cry3Bb1 protein at a concentration of 930 μg/g of diet had no adverse effect on the survival, development, and growth (biomass) of P. chalcites larvae. Furthermore, statistical power analysis indicated that at levels of 80% power and 5% type I error rate, the study design would have been able to detect a minimum 14 (at day 7) to 19% (at days 14, 21, and 28) reduction in survival and a 21 (at day 14) to 18% (at day 28) reduction in biomass of test larvae relative to the negative buffer control groups. Based on the maximum level (93 μg/g) of the Cry3Bb1 protein expressed in MON 863 corn tissues including leaves, roots, and pollen, findings from this study indicate that corn hybrids containing the MON 863 event have a minimum 10 times safety factor for larvae of P. chalcites, and thus pose little risk to this nontarget beneficial insect.