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To isolate gamma-hexachlorocyclohexane (γ-HCH)-degrading bacteria from a single field and to examine their genetic diversity.γ-HCH-degrading bacteria were screened from a long-term experimental field in which γ-HCH has been continuously applied to, and a γ-HCH-degrading sphingomonad strain SS86 was isolated from in 1986. As the result, five strains of sphingomonads were newly isolated. The sequences of several housekeeping genes separated the six strains, including SS86, into two genotypes. Among the genes involved in γ-HCH degradation, the sequences of linC, linD and linE were identical among all six strains, that of linA was identical among five strains, and that of linB was diverse.We calculated that the γ-HCH-degrading populations of the two genotypes arose independently. Not just one but diverse sphingomonads that degrade a particular xenobiotic compound possibly tend to arise and/or accumulate in fields, where that compound has been applied.This study indicates the potential usefulness of a long-term continuous application of xenobiotic compounds to an experimental field in that it would potentially generate diverse micro-organisms able to degrade the compounds.