The evolution of microbial communities with increasing carbon tetrachloride concentrations was studied in two anaerobic columns containing sand and two different clay soils, one of which contained high levels of iron. Microbial communities were characterized through analysis of column effluents with denaturing gradient gel electrophoresis and quantitative polymerase chain reaction for archaea and eubacteria as inlet carbon tetrachloride concentrations were increased from 0.8 to 29 μM. Inhibition of microbial activity was observed in both columns, and was associated with the accumulation of chloroform at concentrations of 0.2 to 0.4 μM as inlet CT concentrations were increased to 2.4-3.0 μM in the low-iron clay column and approximately 16 μM in the iron rich clay column. Inhibition was indicated by decreasing rates of methanol and carbon tetrachloride degradation, decreases in effluent levels of DNA, and shifts in microbial communities of the columns. Even with the inhibition observed, in the iron-rich clay column CT degradation continued to the end of the study with inlet CT concentrations of 29 μM, in contrast to the low-iron clay column in which minimal CT degradation occurred once CT inlet concentrations exceeded 3 μM. The greater capacity for CT degradation in the column containing the ironrich clay was hypothesized to be the result of reaction with biogenic ferrous iron produced by biological dissimilatory iron reduction.