Commercial formulations of hexachlorocyclohexane (HCH) consist of a mixture of four isomers: alpha, beta, gamma, and delta. All four isomers are toxic and recalcitrant pollutants. beta-HCH is more problematic due to its longer persistence in the environment. Sphingomonas sp. BHC-A was able to degrade not only alpha-, gamma-, and delta-HCH but also beta-HCH. To clone a gene responsible for the degradation of beta-HCH, a Tn5 mutation was introduced into BHC-A, and one mutant BHC-A45 defective in beta-HCH degradation was selected. Sequencing analysis showed this mutant had a Tn5 insertion at the site of one haloalkane dehalogenase gene, designated linB2. linB2 was overexpressed in Escherichia coli and the 32-kDa product LinB2 showed the conversion activity of not only beta-HCH to beta-2,3,4,5,6-pentachlorocyclohexanol (beta-PCHL) but also beta-PCHL to beta-2,3,5,6-tetrachloro-1,4-cyclohexanediol.