Many organisms respond to toxic compounds in their environment by inducing regulatory networks controlling the expression and activity of cytochrome P450 monooxygenase (P450s) detoxificative enzymes. In particular, black swallowtail (Papilio polyxenes) caterpillars respond to xanthotoxin, a toxic phytochemical in their hostplants, by activating transcription of the CYP6B1 promoter via several regions located within 150 nt of the transcription initiation site. One such element is the xenobiotic response element to xanthotoxin (XRE-Xan) that lies upstream of consensus XRE-AhR (xenobiotic response element to the aryl hydrocarbon receptor) and OCT-1 (octamer-1 binding site) element known to be utilized in mammalian aryl hydrocarbon response cascades. Two-plasmid transfections conducted in Sf9 cells have indicated that XRE-Xan, XRE-AhR and a number of other proximal elements, but not OCT-1, are critical for basal as well as xanthotoxin- and benzo[alpha]pyrene-induced transcription of the CYP6B1 promoter. Four-plasmid transfections with vectors co-expressing the spineless (Ss) and tango (Tgo) proteins, the Drosophila melanogaster homologues of mammalian AhR and ARNT, have indicated that these proteins enhance basal expression of the CYP6B1 promoter but not the magnitude of its xanthotoxin and benzo[alpha]pyrene induction. Based on these results, we propose that these Drosophila transcription factors modulate basal expression of this promoter in a ligand-independent manner and attenuate its subsequent responses to planar aryl hydrocarbons (benzo[alpha]pyrene) and allelochemicals (xanthotoxin).