Bacterium o-17, a microorganism capable of the ortho dechlorination of 2,3,5,6-polychlorinated biphenyl (PCB), is a member of a sediment-free, nonmethanogenic mixed culture. The culture was examined for the ability to dechlorinate 26 PCB congeners, 12 chlorobenzenes (CBZs), and 6 chlorinated ethenes (CEs). Eight of the PCBs and 4 of the CBZs were dechlorinated including single-flanked ortho PCB chlorines, but double-flanked chlorines of PCBs and CBZs were preferentially dechlorinated. The dechlorination of three of the PCBs (2,3,4,5,6-, 2,3,4,6-, and 2,3,5,6-PCB), three of the CBZs (hexa-, penta-, and 1,2,3-CBZ), and PCE could be sustained for three or more sequential transfers of the bacterial community. Two PCBs (2,3,4- and 2,3,5-PCB), two CBZs (1,2,3,5- and 1,2,4,5-CBZ), and trichloroethene were dechlorinated only when a more extensively chlorinated parent compound was present. Aroclor 1260 and 2,4,6-PCB, not dechlorinated by the culture, inhibited the dechlorination of 2,3,5,6-PCB. Within the culture only bacterium o-17 was linked to dechlorination by PCR-DGGE analysis, confirming that this dehalogenating species was the catalyst for the dechlorination of the compounds tested. The microorganism is capable of dechlorinating several different congeners of PCBs, CBZs, and CEs, and it remains a rare example of an ortho-PCB dechlorinator. However, its limited ability to dechlorinate more extensively chlorinated congeners and Aroclor plus the inhibitory effects of some PCB congeners upon the bacterium is consistent with the observed infrequency of this reaction in the environment. An assessment of bioremediation potential of this microorganism in situ will require a greater understanding of the synergistic, cometabolic and competitive interactions of PCB dechlorinating microbial communities.