Resting cells of 2,4,5-trichlorophenoxyacetic acid-grown Pseudomonas cepacia AC1100 metabolize both dichlorophenols, such as 2,4-dichlorophenol, 2,6-dichlorophenol, 3,4-dichlorophenol, and 3,5-dichlorophenol, and more highly substituted phenols, such as 2,4,6-trichlorophenol and pentachlorophenol, to the corresponding chlorohydroquinones. The first hydroxylation occurs in the para position of the phenol regardless of whether this position is replaced by a chlorine substituent. The first evidence leading to the characterization of para-hydroxylase as a flavin-containing enzyme is provided by the inhibitory effect of methimazole, an alternate substrate for this monooxygenase, on the degradative ability of the strain. In a second step, with tetrachlorohydroquinone, trichlorohydroxyquinone was isolated and completely characterized. Trichlorohydroxyquinone was also obtained from tetrachloroquinone. Incubation of the cells in the presence of an external source of NADPH prevents the further degradation of tetrachlorohydroquinone, suggesting that the quinone derived from the two-electron oxidation of the hydroquinone is more likely the substrate for the second hydroxylation.