none Heliothis virescens (F.) larvae parasitized by Microplitis demolitor Wilkinson gained less weight and molted at sizes below those established for unparasitized larvae. The host stadium from which M. demolitor emerged depended upon the stadium parasitized, and emergence usually occurred about 24 h after the host moved off the diet. Following M. demolitor emergence, H. virescens remained in developmental arrest, neither molting nor displaying any developmental characters associated with metamorphosis. The size of the emerging wasps was correlated with host weight, but primary clutch size was not. M. demolitor laid one to three eggs per host, but usually only one offspring survived to adulthood. That M. demolitor larval development varied with the host stadium parasitized possibly accounts for the variation observed in day of emergence.