Cells of an aerobic three-membered bacterial co-culture, designated as ECO3, capable of cometabolizing and aerobically dechlorinating low-chlorinated biphenyls in the presence of biphenyl, were immobilized on Manville silica beads, on frosted-glass beads and on polyurethane foam cubes in packed-bed bioreactors continuously fed with a biphenyl-saturated air stream. The ECO3 biofilm reactors were found to be capable of extensively mineralizing several pure dichlorobiphenyls (75 mg/l) and Aroclor 1221 (75 mg/l) in batch mode. Immobilized ECO3 cells could aerobically degrade and dechlorinate the dichlorobiphenyls tested more extensively than suspended ECO3 cells. Among the three biofilm reactors, the glass bead bioreactor and the polyurethane bioreactor exhibited the highest capability of mineralizing both dichlorobiphenyls and Aroclor 1221; the polychlorinated biphenyl availability in the bioreactors, more than the biomass availability, both depending on the nature of the support employed, significantly governed the efficiency of the treatment. These results are of interest for the possible development of a bioreactor system for continuous treatment of polychlorinated-biphenyl-contaminated wastewaters.