Spalangia endiusandMuscidifurax raptormultiparasitized house fly pupae. However,S. endiustended to avoid ovipositing in pupae parasitized byM. raptor, whereasM. raptordid not avoid pupae parasitized byS. endius. Behavioral observations indicated that both species could, in some cases, discriminate and subsequently refrain from ovipositing on an already parasitized pupa, but only after drilling into the puparium. Multiparasitism decreased as the time after initial parasitization increased. These findings indicate that the parasitoids use cues, possibly venom, inside the puparium for host discrimination. Multiparasitism should be accounted for in laboratory studies on the reproductive potential of these parasitoids. Furthermore, our results indicate that where these two species are mass released simultaneously, multiparasitism may commonly occur.