Pseudomonas sp. S-47 is capable of catabolizing 4-chlorobenzoate (4CBA) as rarbon and energy sources under aerobic conditions via the mesa-cleavage pathway. 4CBA-dioxygenase and 4CBA-dihydrodiol dehydrogenase (4CBA-DD) catalyzed the degradation af 4CBA to produce 4-chlorocatechol in the pathway. In this study, the xylL gene encoding 4CBA-DD was cloned from the chromosomal DNA of Pseudomonas sp. S-47 and its nucleotide sequence was analyzed. The xylL gene was found to be composed of 777 nucleotide pairs and to encode a polypeptide of 28 kDa with 258 amino acid residues. The deduced amino acid sequence of the dehydrogenase (XylL) from strain S-47 exhibited 98% and 60% homologies with these of the corresponding enzymes, Pseudomonas putida mt-2 (XyIL) and Acinetobacter calcoaceticus (BenD), respectively. However, the amino arid sequences show 30% or less homology with those of Pseudomonas putida (BnzE), Pseudomonas putida Fl (TodD), Pseudomonas pseudoalcaligenes KF707 (BphB), and Pseudomonas sp. C18 (NahB). Therefore, the 4CBA-dihydrodiol dehdrogenase of strain S-47 belongs to the group I dehydrogenase involved in the degradation of mono-aryls with a carboxyl group.