The common parasite species attacking Trichopluisa ni (Lepidoptera: Noctuidae) in southern California were investigated in the laboratory to demonstrate discriminatory abilities. Copidosoma truncatellum (Hymenoptera: Encyrtidae), a polyembryonic, egg-larval parasite, exhibited time-dependent, intraspecific discrimination. The solitary larval parasite, Microplitis brassicae (Hymenoptera: Ichneumonidae), discriminated between unparasitized hosts and those parasitized by Hyposoter exiguae (Hymenoptera: Ichneumonidae) or M. brassicae , but did not discriminate hosts containing C. truncatellum immatures. Multiple parasitization by these two species in hosts containing C. truncatellum resulted in successful development of C. truncatellum in most cases, with some hosts yielding M. brassicae or H. exiguae .