An extractive membrane bioreactor (EMB) is described that used an undefined anaerobic culture to dechlorinate tetrachloroethene (C2Cl4) reductively in a synthetic wastewater. Comparable reactors described in the literature use set-ups where the bacteria are in direct contact with the wastewater, and thus would require the addition of significant quantities of nutrients to the wastewater stream in practical application. In the EMB, a silicone rubber membrane separates the microbial culture from the wastewater stream, so that addition of nutrients can be minimised. The EMB was operated continuously for 48 days and dechlorinated 359 mu mol C2Cl4/(1 biomedium(-1) day(-1)) on average. Lactate was fed as an electron donor and C2Cl4 dechlorination was verified by chloride measurements. Particular attention was paid to the reduction of transmembrane C2Cl4 flux caused by a membrane-attached biofilm. Following a start-up period, the reactor operation was stable and remained largely unaffected by biofilm thickness and oxygen contamination from the wastewater.