The larval food range of the black swallowtail, Papilio polyxenes, is restricted naturally to plants of the family Umbelliferae. When larvae were reared on celery leaves cultured in solutions of sinigrin (a chemical occurring in various species of Cruciferae), feeding rates were not significantly affected but growth and development were substantially reduced. At concentrations of 0.1% (fresh weight of leaf or higher, sinigrin, caused 100% larval mortality. These findings suggest that the larval feeding niche of P. polyxenes is bounded in part by chemical barriers of a toxicological as well as a behavioral nature. They also provide further evidence that sinigrin and other mustard oil glucosides serve a defensive function in plants of the family Cruciferae.