A novel aerobic pentachloronitrobenzene-degrading bacterium, Nocardioides sp. strain PD653, was isolated from an enrichment culture in a soil-charcoal perfusion system. The bacterium also degraded hexachlorobenzene, a highly recalcitrant environmental pollutant, accompanying the generation of chloride ions. Liberation of (14)CO(2) from [U-ring-(14)C] hexachlorobenzene was detected in a culture of the bacterium and indicates that strain PD653 is able to mineralize hexachlorobenzene under aerobic conditions. The metabolic pathway of hexachlorobenzene is initiated by oxidative dechlorination to produce pentachlorophenol. As further intermediate metabolites, tetrachlorohydroquinone and 2,6-dichlorohydroquinone have been detected. Strain PD653 is the first naturally occurring aerobic bacteria capable of mineralizing hexachlorobenzene.