The effect of moisture content an the reductive dechlorination of polychlorinated biphenyls and population dynamics of dechlorinating microorganisms was investigated in sediments spiked with Aroclor 1248. In sediment slurry with an overlying water layer, dechlorination ensued after a 4-week lag period and reduced the average number of chlorines per biphenyl from 3.91 to 3.15 after 48 weeks. In the sediments of reduced moisture content, however, dechlorination occurred after a lag period of 12 weeks and decreased the average number of chlorines per biphenyl to only 3.62, and the dechlorination rate was also slower. When the population size of dechlorinators, methanogens, and sulfate-reducing bacteria was determined by the most probable number techniques, however, no difference was found between the slurry and the low-moisture sediments, except for methanogens. The growth of dechlorinating populations coincided with the end of the lag period and they then increased by 3 orders of magnitude in two conditions. Specific growth rate of dechlorinators showed little difference between the slurry and the low-moisture sediments; however, growth yield was high in the sediments of reduced moisture content. The reduction of sediment moisture decreased the dechlorination rate and extent of PCBs but did not inhibit the growth of PCB dechlorinators.