The Peruvian stick insect Oreophoetes peruana is the only known animal source for unsubstituted quinoline in nature. When disturbed, these insects discharge a defensive secretion containing quinoline. Analysis of samples obtained from L-[2′,4′,5′,6,′7′-2 H 5 ]tryptophan-fed stick insects demonstrated that the insects convert it to [5,6,7, H 4 ]quinoline by removing the 2′-CH moiety in the indole ring of tryptophan. Analogous experiments using L-[1′-15 N]tryptophan and L-[1′-15 N, 15 NH 2 ]tryptophan showed that the indole-N atom is retained while the α-amino group is eliminated during the biosynthesis. Mass spectra recorded from quinoline derived from [2-13 C 1 ]tryptophan-fed insects indicated that the αcarbon atom of tryptophan is incorporated as the C-2 atom of the quinoline ring.