The dechlorination of carbon tetrachloride (CCl4) by free-living and attached bacteria under anaerobic conditions was studied to examine the relationship between porous media and electron donor. Two batch-type experiments, the free-living and attached bacterial systems, were conducted with and without addition of 0.5-mm glass beads. Glucose and acetate were selected as the primary electron donors because they are easily biodegradable. Direct epifluorescence technology, the DAPI (4’ 6-diamidino -2-phenylindole) method, was used for counting the microbial activities. Adding glass beads could accelerate the dechlorination rate of CCl4. Removals of 44%-57% were observed in free-living bacterial system. Whereas a two- to fivefold increase in the CCl4 dechlorination rate was observed in the attached system. Experimental results and thermodynamic calculations indicated that glucose is a better supplementary substrate than acetate for stimulating the dechlorinating capability of microorganisms because of its relatively high available free energy. A higher concentration of substrate provided more reducing power for attached bacteria to initiate the dechlorination reaction. The pseudo-first-order rate constants of CCl4 dechlorination ranged from 0.007 day(-1) to 0.017 day(-1) and from 0.011 day(-1) to 0.0625 day(-1) for free-living and attached bacterial systems respectively. Microscopic observation revealed a three- to eightfold difference of microbial number between the free-living and attached bacterial systems. On the basis of the results in this study, we can conclude that the presence of porous media and an electron donor can change the dechlorination capabilities of the microorganisms. This work will be valuable in the design of in situ bioremediation as it discusses the specific area of the medium and supplementation with an electron donor to stimulate the indigenous microflora.