A strict anaerobic bacterium, strain Y51, was isolated from soil contaminated with tetrachloroethene (PCE). Strain Y51 is capable of very efficiently dehalogenating PCE via trichloroethene (TCE) to cis-1,2-dichloroethene (cis-1,2-DCE) at concentrations as high as 960 microM and as low as 0.6 microM. Strain Y51 was gram-negative, motile with some lateral flagella, and curved rod-shaped. On the basis of the 16S rDNA sequence, the organism was identified to be a species within the genus Desulfitobacterium. Strain Y51 also had dehalogenation activities toward polychloroethanes such as hexa-, penta-, and tetrachloroethanes, from which dichloroethenes were produced as the final products. The cell extracts mediated the dehalogenation of PCE with reduced methyl viologen as an electron carrier at the specific rate of 5.0 nmol min(-1) mg cell protein(-1) (pH 7.2, 37 degrees C). Dehalogenation was highly susceptible to air oxidation, and to potential alternative electron acceptors such as nitrite or sulfite.