1,2-Dihydroxynaphthalene dioxygenase was purified to homogeneity from a bacterium that degrades naphthalenesulfonic acids (strain BN6). The enzyme requires Fe2" for maximal activity and consists of eight identical subunits with a molecular weight of about 33,000. Analysis of the NH2-terminal amino acid sequence revealed a high degree of homology (22 of 29 amino acids) with the NH2-terminal amino acid sequence of 2,3-dihydroxybiphenyl dioxygenase from strain Pseudomonas paucimobiis Ql. 1,2-Dihydroxynaphthalene dioxygenase from strain BN6 shows a wide substrate specificity and also cleaves 5-, 6-, and 7-hydroxy-1,2-dihydroxynaphthalene, 2,3-and 3,4-dihydroxybiphenyl, catechol, and 3-methyl-and 4-methylcatechol. Similar activities against the hydroxy-1,2-dihydroxynaphthalenes were also found in cell extracts from naphthalenedegrading bacteria.Amino-and hydroxynaphthalenesulfonic acids (ANS and HNS, respectively) serve as building blocks for the largescale synthesis of azo dyes. Since arylsulfonates are very rare among natural compounds (21), naphthalenesulfonic acids are referred to as xenobiotics. Arylsulfonates are major pollutants of the environment, and the contamination of the Rhine River by sulfur-organic compounds is largely due to this class of compounds (25). Nevertheless, bacteria which degrade naphthalenesulfonic acids have repeatedly been isolated (4, 5, 29-31, 41, 43). Brilon et al. (4, 5) selected pseudomonads which degraded naphthalene-2-sulfonic acid (2NS) and naphthalene-1-sulfonic acid (1NS). The initial reaction in the degradation of iNS and 2NS is catalyzed by a 1,2-dioxygenase which weakens the carbon-sulfur bond.