The CYP6B1 and CYP6B3 cytochrome P450 monooxygenases in the midgut of the black swallowtail participate in the metabolism of toxic furanocoumarins present in its host plants. In this study, biochemical analyses indicate that the fat body metabolizes significant amounts of the linear furanocoumarins bergapten and xanthotoxin after larvae feed on xanthotoxin. Northern analyses of the combined CYP6B1/3 transcript expression patterns indicate that transcripts in this P450 subfamily are induced in the midgut and fat body by xanthotoxin. Semi-quantitative RT-PCR analyses of individual CYP6B1/CYP6B3 mRNAs indicate that CYP6B1 transcripts are induced by xanthotoxin in all tissues examined and that CYP6B3 transcripts are induced in the fat body only. These results indicate that the fat body participates in the P450-mediated metabolism of excess furanocoumarins unmetabolized by the midgut. Although transcripts of both genes were detected and CYP6B1 transcripts were induced by xanthotoxin in the integument, furanocoumarin metabolism was not detected. Comparison of these P450 promoters with the promoters of alcohol dehydrogenase genes expressed in the fat bodies of several Drosophila species suggest that the xanthotoxin inducibilities of these P450 genes in fat bodies are regulated by elements other than those modulating expression of Adh genes.