The population dynamics of a mixed microbial culture dechlorinating trichloroethene (TCE), cis-1,2-dichloroethene (cDCE), 1,2-dichloroethane (1,2-DCA), and vinyl chloride (VC) to ethene were studied. Quantitative PCR revealed that Dehalococcoides, Geobacter, Sporomusa, Spirochaetes, and Methanomicrobiales phylotypes grew in short-term experiments. Both Geobacter and Dehalococcoides populations grew during TCE dechlorination to cDCE, but only Dehalococcoides populations grew during further dechlorination to ethene. The cell yields for Dehalococcoides determined in this study were similar on an electron equivalent basis regardless of the chlorinated compound transformed: (0.9+/-0.3) x 10(8)16S rRNA gene copies/microelectron equivalent (microeeq) ethene produced during cDCE dechlorination, (1.5 +/-0.3) x 10(8) copies/microeeq ethene produced during VC dechlorination, and (1.6+/-0.8) x 10(8) copies/ u,eeq ethene produced during 1,2-DCA dihaloelimination. The yield for the Geobacter population on TCE was estimated to be (1+/-0.5) x 10(8) copies/microeeq cDCE produced. Calculations showed that the Geobacter population was likely responsible for approximately 80% of the TCE dechlorinated to cDCE in this experiment. Acetogenesis by a Sporomusa population was the main competition to dechlorination for reducing equivalents. Sporomusa did not transform any chlorinated substrates tested, but was capable of converting methanol to acetate and hydrogen for dechlorination. Understanding the functions of various populations in mixed communities may explain why Dehalococcoides spp. are active at some sites and not others, and may also assist in optimizing the growth of bioaugmentation cultures, both in the laboratory and in the field.