A rod shaped, gram positive, non sporulating Acetobacterium strain was isolated that dechlorinated 1,2dichloroethane (1,2-DCA) to ethene at a dechlorination rate of up to 2 nmol Cl- min(-1) mg(-1) of protein in the exponential growth phase with formate (40 mM) as the substrate. Although with other growth substrates such as pyruvate, lactate, H2/CO2, and ethanol higher biomass productions were obtained, the dechlorination rate with these substrates was more than 10-fold lower compared with formate growing cells. Neither cell extracts nor autoclaved cells of the isolated Acetobacterium strain mediated the dechlorination of 1,2-DCA at significant rates. The addition of 1,2-DCA to the media did not result in increased cell production. No significant differences in corrinoid concentrations could be measured in cells growing on several growth-substrates. However, these measurements indicated that differences in corrinoid structure might cause the different dechlorination activity. The Acetobacterium sp. strain gradually lost its dechlorination ability during about 10 transfers in pure culture, probably due to undefined nutritional requirements. 16S rDNA analysis of the isolate revealed a 99.7% similarity with Acetobacterium wieringae. However, the type strains of A. wieringae and A. woodii did not dechlorinate 1,2-DCA.