Platygaster robiniae Buhl and Duso (Hymenoptera: Platygastridae) is a gregarious koinobiont endoparasitoid of Obolodiplosis robiniae (Haldeman) (Diptera: Cecidomyiidae). The life cycle and development of the parasite is described. P. robiniae proves to be an egg-larval parasitoid, parasitizing eggs of O. robiniae and emerging from host larvae, and eggs of the wasp develop monembryonically. Its egg and embryonic stages are prolonged to some extent. The parasitoid undergoes only one larval instar, and total development time from egg to adult takes about 28 days to complete. The life cycle of a parasitoid generation is perfectly synchronized with that of the host: the waps emerges near the emergence time of the host, so that they can parasitize host eggs.