Prepupae of Hypera eximia (LeConte) (Coleoptera: Curculionidae), a natural host, and prepupae of factitious insect hosts were tested for their physiological responses to stinging by the gregarious ectoparasite, Necremnus breviramulus Gahan (Hymenoptera: Eulophidae), and to injection with an extract of the venom gland tissue dissected from the lower reproductive tract of the female parasite. The arrestment of development produced by N. breviramulus venom was expressed in the natural host, H. eximia, and in all insects tested outside the natural host range of the parasite (Coleoptera: Curculionidae, Cerambycidae, and Chrysomelidae; Lepidoptera: Plutellidae and Noctuidae). Arrestment activity was found to be associated with the aqueous extract of the venom apparatus, and the response was shown to be dose-dependent. The protein composition of the venom from N. breviramulus differed from other eulophids tested and did not contain the molt-arresting protein found in Euplectrus spp. The difference in venom proteins may account for the different physiological effects and host range of these eulophid parasites.