Results are presented from a chemostat study where the reductive dehalogenation of PCE was evaluated in the absence and presence of sulfate. Two chemostats inoculated with the Point Mugu culture, which contains strains of Dehalococcoides mccartyi, were operated at a 50 day HRT and fed PCE (1.12 mM) and lactate (4.3 mM). The control chemostat (PM-5L, no sulfate), achieved pseudo-steady-state transformation of PCE to ethene (98%) and VC (2%) at 2.4 nM of H(2). Batch kinetic tests with chemostat harvested cells showed the maximum rate (k(max)X) value for each dehalogenation step remained fairly constant, while hupL clone library analyses showed maintenance of a diverse D. mccartyi community. Sulfate (1 mM) was introduced to the second chemostat, PM-2L. Effective sulfate reduction was achieved 110 days later, resulting in 600 μM of total sulfide. PCE dechlorination efficiency decreased following complete sulfate reduction, yielding ethene (25%), VC (67%), and cis-DCE (8%). VC dechlorination was most affected, with k(max)X values decreasing by a factor of 50. The decrease was associated with the enrichment of the Cornell group of D. mccartyi and decline of the Pinellas group. Long-term exposure to sulfides and/or competition for H(2) may have been responsible for the community shift.