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'-2H5]tryptophan-fed stick insects demonstrated that the insects convert it to [5,6,7,8-2H4]quinoline by removing the 2'-CH moiety in the indole ring of tryptophan. Analogous experiments using l-[1'-15N]tryptophan and l-[1'-15N,15NH2]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 [2-13C1]tryptophan-fed insects indicated that the α-carbon atom of tryptophan is incorporated as the C-2 atom of the quinoline ring.