The corn earworm, Heliothis zea (Boddie), the tobacco budworm, H. virescens (F.), and H. subflexa (Gueneé) were tried and found to be suitable hosts for Microplitis crocepes (Cresson). Other species tried were found unsuitable either for oviposition or development of the parasite. Given a choice of instars of H. zea , the parasite attacked the 3rd instar most efficiently. However, all instars up to the 5th were equally suitable for development of the parasite once oviposition occurred. Variations in the growth of H. zea larvae parasitized in different instars were examined.