Screening studies with strict and facultative anaerobic bacteria showed that Clostridium app. and several other representatives of Bacillaceae and Enterobacteriaceae actively degraded gamma-hexachlorocyclohexane (gamma-HCH) under anaerobic conditions. Representatives of Lactobacillaceae and Propronibacterium were inactive. With 36Cl-labelled gamma-HCH a nearly complete dechlorination was shown to occur in 4--6 days by Clostridium butyricum, C. pasteurianum and Citrobacter freundii, while other facultative anaerobic species were less active. Aerobically grown facultative anaerobes also dechlorinated actively gamma-HCH during subsequent anaerobic incubation with glucose, pyruvate or formate as substrates. The alpha-, beta- and delta-HCH isomers were also, but more slowly, dechlorinated (gamma larger than alpha larger than beta larger than or equal to delta-HCH). All species active in anaerobic degradation of gamma-HCH formed gamma-tetrachlorocyclohexene (TCH) as the main intermediate metabolite and no gamma-pentachlorocyclohexene (PCH) or other isomers of TCH or PCH have been found. Small amounts of tri- and tetrachlorinated benzenes have been found too. The mechanism of dechlorination is discussed.