This study was conducted to examine the application of the anaerobic bacterium Dehalospirillum multivorans in a biofilm reactor for the reductive dechlorination of tetrachloroethene (PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (DCE). A laboratory scale fluidized-bed reactor, operated under continuous flow conditions at 20°C, converted PCE to TCE at an apparent maximum reaction rate of 55 nmol/min/mg protein and TCE to DCE at 90 nmol/min/mg protein. These high rates should be even higher with an improved reactor performance since the apparent maximum reaction rates of the suspended cells (30°C) were by a factor of about 3.5 higher. With respect to its technical application D. multivorans is quite insensitive against unfavourable conditions, e.g. the presence of oxygen and low temperature. Natural groundwater components like nitrate and sulfate did not interfere with the dechlorination process up to a concentration of 5 mmol/l. Possible co-contaminants like chlorinated methanes, however, strongly inhibited PCE-dechlorination of the suspended cells, whereas chlorinated ethanes had no influence up to a concentration of 800 μmol/l. Experiments are presently being carried out to study the influence of biofilm immobilization on the protection of the dechlorination process against organic and inorganic inhibitors.