A commensal system of the aerobic methanotroph Methylosinus sporium with anaerobic methanogens was obtained using cell coimmobilization in a chitasan-lignosulfonate matrix and applied for the dechlorination of tetrachloroethylene (PCE) under coupled anaerobic/aerobic conditions. Due to the oxygen gradient in a bean of immobilized cells, anaerobic transformation of PCE into lower chlorinated compounds was coupled with aerobic dechlorination of these intermediates. At a PCE loading rate of 42.9 mg PCE L-R(-1) day(-1) and an oxygenation rate of 4.9 l O-2/L-R(-1) day(-1), the mineralization efficiency of the commensal system was 88% as opposed to 38% in the control system which was not inoculated with the methanotrophs. The dechlorination rates were 6.7 and 3.0 mg PCE g(-1) VSS day(-1) for the commensal and central systems, respectively.