none The braconid wasp, Cardiochiles nigriceps, successfully parasitizes the host larva, Heliothis virescens, by inhibiting host pupation. The parasitoid suppresses the function of the prothoracic gland and its competence to respond to prothoracicothopic hormone (PTTH) after larval-pupal commitment caused by a small surge of ecdysteroid. Prothoracic gland from both unparasitized normal and parasitized larvae before the small surge responded to PTTH in vitro by releasing ecdysteroid, whereas after this surge only the prothoracic gland of the unparasitized larvae released the ecdysteroid in the presence of PTTH. Both calyx and venom fluid of female wasps, which are injected into the host with the eggs at the time of oviposition, were required to depress prothoracic gland function. Prothoracic glands pre-incubated in vitro for 6 h with body calyx and venom fluids resulted in inhibiting the release of ecdysteroid by pupally committed prothoracic gland. The results suggest that the calyx and venom fluids are directly able to depress the function of the prothoracic gland and the competence of prothoracic gland to respond to PTTH.