Phenanthrenequinone (PQ), which occurs widely as a pollutant and as a major metabolite of phenanthrene in a number of species, has been demonstrated to undergo futile redox cycling leading to oxidative stress. In the presence of cytosolic fractions of selected channel catfish tissues, PQ undergoes enzymatic reduction which is mediated by either NADH or NADPH and is composed of dicoumarol-sensitive and -insensitive components. Most notably, gastric cytosol catalyzed a disproportionately high level of NADPH-dependent, dicoumarol-sensitive PQ reduction as compared to gill, liver, and kidney cytosols. In the presence of stomach cytosol and NADPH, PQ facilitated production of superoxide anion at rates several fold higher than those mediated by menadione. The dicoumarol-sensitive PQ-reducing agent, which we have termed NADPH: phenanthrenequinone oxidoreductase (PQR), was purified by affinity chromatography and was demonstrated to be separable from DT diaphorase activity in gastric cytosol. Under aerobic conditions, purified PQR facilitates redox cycling of PQ as indicated by continued NADPH oxidation and hydrogen peroxide production. Under anaerobic conditions, NADPH oxidation is limited to a quantity indicative of PQ reduction to the hydroquinone. Substrate specificities, pH profiles, and kinetic characteristics combine to indicate that PQR represents a novel quinone reductase in this species.