The use of gamma-hexachlorocyclohexane (HCH) as a terminal electron acceptor via organohalide respiration was demonstrated for the first time with an enrichment culture grown in a sulfate-free HEPES-buffered anaerobic mineral salts medium. The enrichment culture was initially developed with soil and ground-water from an industrial site contaminated with HCH isomers, chlorinated benzenes, and chlorinated ethenes. When hydrogen served as the electron donor, 79-90% of the electron equivalents from hydrogen were used by the enrichment culture for reductive dechlorination of the gamma-HCH, which was provided at a saturation concentration of approximately 10 mg/L Benzene and chlorobenzene were the only volatile transformation products detected, accounting for 25% and 75% of the gamma-HCH consumed (on a molar basis), respectively. The enrichment culture remained active with only hydrogen as the electron donor and gamma-HCH as the electron acceptor through several transfers to fresh mineral salts medium for more than one year. Addition of vancomycin to the culture significantly slowed the rate of gamma-HCH dechlorination, suggesting that a Gram-positive organism is responsible for the reduction of gamma-HCH. Analysis of the gamma-HCH dechlorinating enrichment culture did not detect any known chlororespiring genera, including Dehalobacter. In bicarbonate-buffered medium, reductive dechlorination of gamma-HCH was accompanied by significant levels of acetogenesis as well as methanogenesis. (C) 2011 Elsevier B.V. All rights reserved.