The principal metabolite detected in the excreta of chickens treated orally with 3-phenoxybenzoic acid is alpha-N-acetyl-delta-N-(3-phenoxybenzoyl)-L-ornithine. This conjugate has been isolated and purified by preparative HPLC and its structure authenticated by mass and NMR spectrometry. Although diacyl ornithine conjugates of aromatic carboxylic acids are formed in some bird species, monosubstituted derivatives are rare. Moreover, this is the first example of the occurrence of an N-acetylornithine conjugate in the biotransformation of a xenobiotic in animals. The conjugation of such nonnutritive acids in avian species is discussed.