The parasitic wasp Microplitis croceipes (Cresson) (Hymenoptera: Braconidae) learns to associate odors with food resources and subsequently exhibits a characteristic food-seeking behavior when encountering the learned odor. Wasps so conditioned, learned and subsequently demonstrated an ability to distinguish among aflatoxigenic and non-aflatoxigenic Aspergillus flavus and A. parasiticus strains. The effects of fungal species, strain, age (5, 10-12, 20, and 30 d) and growth media (potato dextrose agar, peanut agar and corn agar) on the learning and recognition responses of the conditioned wasps were examined. The level of differentiation between fungal strains by conditioned wasps was lowest when working with 5-d-old fungal cultures but increased with age and generally peaked with 20-d-old fungi. Wasps responded generally stronger to the fungal strain conditioned to independent of growth media. This ability of parasitic wasps to learn and distinguish fungal odors can open new avenues in insect learning. Helicoverpa zea (Noctuidae) : Host Heliothis virescens (Noctuidae) : Host Microplitis croceipes (Braconidae) : Parasite Sensory reception; Behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Abiotic factors Helicoverpa zea; Heliothis virescens; Hymenopteran parasites; Parasite differentiation of fungal pathogen odours associated with host food plants; Microplitis croceipes Microplitis croceipes; Chemoreception; Differentiation of fungal pathogen odours associated with food plants of leipdopteran hosts; Learning; Lepidopteran hosts; Helicoverpa zea & Heliothis virescens; Chemical factors none