Anaerobic tetrachloroethene(C2-Cl-4)-dechlorinating bacteria were enriched in slurries from chloroethene-contaminated soil. With methanol as electron donor, C2Cl4 and trichloroethene (C2HCl3) were reductively dechlorinated to cis-1,2-dichloroethene (cis-C2H2Cl2), whereas, with L-lactate or formate, complete dechlorination of C2Cl4 via C2HCl3, cis-C2H2Cl2 and chloroethene (C2H3Cl) to ethene was obtained. In oxic soil slurries with methane as a substrate, complete cometabolic degradation of cis-C2H2Cl2 was obtained; whereas C2HCl3 was partially degraded. With toluene or phenol both of the above were readily co-metabolized. Complete degradation of C2Cl4 was Obtained in sequentially coupled anoxic and oxic chemostats, which were inoculated with the slurry enrichments. Apparent steady states were obtained at various dilution rates (0.02-0.4 h(-1)) and influent C2Cl4-concentrations (100-1000 mu M). In anoxic chemostats with a mixture of formate and glucose as the carbon and electron source, C2Cl4 was transformed at high rates (above 140 mu mol l(-1) h(-1) corresponding to 145 nmol Cl- min(-1) mg protein(-1)) into cis-C2H2Cl2 and C2H3Cl. Reductive dechlorination was not affected by addition of 5 mM sulphate, but strongly inhibited after addition of 5 mM nitrate. Our results (high specific dechlorination rates and loss of dechlorination capacity in the absence of C2Cl4) suggest that C2Cl4-dechlorination in the anoxic chemostat was catalysed by specialized dechlorinating bacteria. The partially dechlorinated intermediates, cis-C2H2Cl2 and C2H3Cl, were further degraded by aerobic phenol-metabolizing bacteria. The maximum capacity for chloroethene (the sum of tri-, di-and monochloro derivatives removed) degradation in the oxic chemostat was 95 mu mol l(-1) h(-1) (20 nmol min(-1) mg protein(-1)), and that of the combined anoxic --> oxic reactor system was 43.4 mu mol l(-1) h(-1). This is significantly higher than reported thus far.