How polyphagous herbivores up-regulate their counterdefense genes in response to a broad range of structurally different allelochemicals remains largely unknown. To test whether this is accomplished by having more allelochemical-response elements or the similar number of functionally more diverse elements, we mapped out the cis-acting elements mediating the induction of the allelochemical-metabolizing CYP321A1 from the generalist Helicoverpa zea by xanthotoxin and flavone, two structurally distinct allelochemicals with very different encounter rate by this species. Seven xanthotoxin-responsive elements were localized by analyzing promoter activities of varying length of CYP321A1 promoter in H. zea fatbody cells. Compared with the 5 flavone-responsive elements mapped out previously, there are four common elements (1 essential element, 2 enhancers, and 1 negative element) mediating induction of CYP321A1 by both of the two allelochemicals. The remaining four elements (3 enhancers and 1 negative element), however, only regulate induction of CYP321A1 by either of the two allelochemicals. Co-administration of the two allelochemicals resulted in an induction fold that is significantly lower than the expected additive value of the two allelochemicals. These results indicate that xanthotoxin- and flavone-induced expressions of CYP321A1 are mediated mainly by the functionally more diverse common elements although the allelochemical-unique elements also play a role.