β-Hexachlorocyclohexane (β-HCH) was microbiologically transformed under anaerobic conditions by methanogenic granular sludges from upflow anaerobic sludge blanket (UASB) reactors fed with methanol, volatile fatty acids (VFA), or sucrose as substrates. These sludges, which had not priorily been exposed to β-HCH transformed β-HCH to benzene and chlorobenzene. Usually 2-3 times as much benzene as chlorobenzene was formed. The transformation rates ranged from 0.37 to 0.46 β-HCH (g of volatile suspended solids (VSS)) -1 day -1 at 30 °C. β-HCH was not transformed by autoclaved (sterile) sludge, indicating the biotic nature of the reaction. βand also R-HCH present in contaminated soil were both found to be converted to chlorobenzene and benzene upon incubation of the soil under anaerobic conditions. Equal amounts of benzene and chlorobenzene were formed. The results show that β-HCH transforming bacteria are present in different anaerobic environments. This finding may be of importance for the application of anaerobic bioremediation on sites contaminated with HCH isomers.