The degradation of p-nitrophenol (PNP) by Moraxella and Pseudomonas spp. involves an initial monooxygenase-catalyzed removal of the nitro group. The resultant hydroquinone is subject to ring fission catalyzed by a dioxygenase enzyme. We have isolated a strain of an Arthrobacter sp., JS443, capable of degrading PNP with stoichiometric release of nitrite. During induction of the enzymes required for growth on PNP, 1,2,4-benzenetriol was identified as an intermediate by gas chromatography-mass spectroscopy (GC-MS) and radiotracer studies. 1,2,4-Benzenetriol was converted to maleylacetic acid, which was further degraded by the beta-ketoadipate pathway. Conversion of PNP to 1,2,4-benzenetriol is catalyzed by a monooxygenase system in strain JS443 through the formation of 4-nitrocatechol, 4-nitroresorcinol, or both. Our results clearly indicate the existence of an alternative pathway for the biodegradation of PNP.