I examined the effects of host age at oviposition, from 0 to 4 days (egg to mature larva), on emergence rate and developmental period of the leafminer parasitoid Gronotoma micromorpha (Perkins) (Hymenoptera: Eucoilidae) under laboratory conditions in the host Liriomyza trifolii (Burgess) (Diptera: Agromyzidae). No significant difference was found in the emergence rates of G. micromorpha among the host ages examined. However, the developmental period of G. micromorpha was significantly shorter when ovipositing into 3- or 4-d-old L. trifolii. The occurrence of thelytokous parthenogenesis in G. micromorpha was confirmed. The results show that G. micromorpha is an egg-pupal and larval-pupal parasitoid, utilizing a wide range of host stages in which oviposition can occur and progeny can complete development.