The polyphagous corn earworm Helicoverpa zea frequently encounters aflatoxins, mycotoxins produced by the pathogens Aspergillus flavus and A. parasiticus that infect many of the herbivore’s hostplants. Given that ingestion of aflatoxins by H. zea is concurrent with its consumption of hostplants with a diversity of phytochemicals, we examined the effects of phytochemicals on the metabolism of aflatoxin B1 (AFB1) by this herbivore. AFB1 turnover was not detected with midgut proteins isolated from fifth instar larvae feeding on control diets. However, transformation of AFB1 was observed for midgut proteins isolated from larvae feeding on diets supplemented with xanthotoxin (XAN), coumarin (COU) and indole-3-carbinol (I3C). The main metabolite identified in larval midguts was aflatoxin P1, an O-demethylated product of AFB1; this same product was generated by CYP321A1, a P450 induced by these three phytochemicals. The magnitude of CYP321A1 induction by these chemicals correlates well with the relative induced metabolic activities (COU>XAN>I3C). These results indicate that reduction of toxicity by these phytochemicals found in our previous studies can be achieved by induction of CYP321A1.