To clarify the interaction among the European corn borer,Ostrinia nubilalis, the parasitic wasp,Macrocentrus grandii, and the entomopathogenNosema pyrausta, we examined behavior and survival in corn stems ofO. nubilalislarvae infected withN. pyraustaand parasitized byM. grandii.Fifty-four percent of heavily infected, parasitizedO. nubilalislarvae emigrated from their tunnels before emergence of parasitoid larvae and did not reestablish on corn plants. One-third as many heavily infected host larvae as noninfected larvae remained in tunnels until parasitoid larval emergence. The tunnels bored by infected, parasitizedO. nubilalislarvae did not differ in length from those of noninfected parasitized larvae, suggesting that emigration behavioral changes occur after feeding. Parasitoid larval and pupal mortalities were greater in infectedO. nubilalishosts than in noninfected hosts. Greater than 46% of heavily infectedM. grandiibroods wandered instead of remaining clustered around host carcasses and pupating. This behavior accounted for almost ≈40%of the observed mortality in infected parasitoids. Wandering was attributed to decreased production of cocoons by hosts, causing parasitoid larvae to search for suitable substrates on which to spin their own cocoons. Percentage emergence ofM. grandiiadults from corn stalks was significantly lower inN. pyrausta-infected treatments, but the sex ratio of the parasitoid was unaffected by the level of host infection. The mean number ofM. grandiiadults per brood was reduced from 39.8 in the noninfected treatment to 10.5 in heavily infected larvae.