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The cytochrome P-450 monooxygenase system has been implicated in plant utilization by at least three species of Drosophila (D. nigrospiracula, D. mettleri, and D. mojavensis) that am endemic to the Sonoran Desert of the southwestern United States and northwestern Mexico. Basal and induced levels of total cytochrome P-450 were determined for third-instar and decapitated 2- to 5-day post eclosion adults of the three desert species. Total P-450 levels, both basal and induced for all species assayed, were significantly higher for adults than for larvae by up to 20-fold. On a per organism basis, the levels of in vitro metabolism of the cactus alkaloid, carnegine, and patterns of response to induction by cactus tissue for adult desert Drosophila approximated those of larvae. Induction by phenobarbital, however, resulted in levels of in vitro carnegine metabolism that were up to 5.6-fold higher in adults than in larvae.