Technical Abstract: Within the genus Papilio, the P. glaucus group contains the most polyphagous species within the Papilionidae. The majority of species are associated with hostplants in the families Rutaceae and Apiaceae, and characterizing most are secondary metabolites called furanocoumarins. Recent phylogenetic studies suggest that furanocoumarin metabolism is an ancestral trait, with the glaucus group derived from ancestors associated with furanocoumarin-containing Rutaceae. In this study, we examined this relationship by conducting a gravimetric analysis of growth that used various concentrations of the furanocoumarin xanthotoxin. Papilio multicaudatus, the putative ancestor of the glaucus group, includes at least one furanocoumarin-containing rutaceous species among its hostplants; this species can consume leaf tissue containing up to 0.3% xanthotoxin with no detectable effect on relative growth rate, relative consumption rate, or efficiency of conversion of ingested food. As is the case for other Papilio species, xanthotoxin metabolism is mediated by cytochrome P450 monooxygenases (P450s). Ingestion of xanthotoxin by ultimate instar P. multicaudatus increases activity up to 30- fold in a dose-dependent fashion. Midguts of induced larvae can also effectively metabolize six other furanocoumarins, including both linear (bergapten, isopimpinellin, imperatorin) and angular (angelicin, sphondin) forms. A metabolite of xanthotoxin in the frass from xanthotoxin-treated larvae, identified as 6-(7-hydroxy-8-methoxycoumaryl)-acetic acid by MS¿ MS and NMR analyses, is identical to one from the frass of P. polyxenes. The occurrence of this metabolite in two swallowtails and the presence of a second metabolite of xanthotoxin, 6-(7-hydroxy-8-methoxycoumaryl)-hydro-xyethanol in the frass of both P. polyxenes and Depressaria pastinacella are consistent with the suggestion that lepidopterans share as the first step of xanthotoxin metabolism the P450-mediated epoxidation of the furan ring double bond.