CYP6B4, a cytochrome P450 gene from the tiger swallowtail Papilio glaucus, is transcriptionally induced in the midgut by dietary furanocoumarins, plant allelochemicals that can crosslink DNA in their UV-activated form. The CYP6B4 promoter contains an overlapping EcRE/ARE/XRE-xan element similar to that used for basal and xanthotoxin-inducible expression of the CYP6B1 promoter from the black swallowtail Papilio polyxenes. Transfection of the CYP6B4 promoter:CAT reporter construct into Sf9 cells demonstrates that the basal and xanthotoxin-inducible expression levels observed reflect the relative expression levels of this gene in the midguts of tiger swallowtail larvae. Transfections of mutant CYP6B4 promoter constructs into Sf9 cells indicate that the EcRE/ARE/XRE-xan element is necessary for CYP6B4 induction by xanthotoxin but not for its minimal basal expression. In addition to these elements, the CYP6B4 and CYP6B1 promoters also contain putative XRE-AhR elements identical to the aryl hydrocarbon response elements present in mammalian phase I detoxification genes. Transfections of CYP6B4 and CYP6B1 promoters containing EcRE/ARE/XRE-xan and XRE-AhR elements indicate that both are induced significantly by benzo(alpha)pyrene, an aryl hydrocarbon widespread in the environment, as well as by xanthotoxin, an allelochemical encountered in their hostplants.