1,1,1-Trichloroethane (TCA), a major organic and groundwater contaminant, has very strong toxic effects on humans, plants and microorganisms. Effects of TCA on enzymatic activity and microbial diversity were investigated in the anaerobic sequencing batch reactor (ASBR) under methanogenic, nitrate-reducing, sulfate-reducing and benzene/toluene degrading conditions. The activities of three enzymes (lactate dehydrogenase, phosphatase and protease) were significantly decreased in the presence of 5 mg/L TCA. Within these three affected enzymes, phosphatase activity may serve as a noteworthy marker of bacterial toxicity. The activity of phosphatase was 0.2 U/L in methanogenic conditions with 5 mg/L TCA, which was 99% lower than the controls, and the enzyme activity was 18.6 U/L in methanogenic conditions with 1 mg/L TCA, which was 7% lower than the controls. DGGE profiles showed that TCA altered the bacterial community distribution and diversity obviously during the 21 day of TCA exposure. The enzyme activities decreased second lowest but TCA degrading strains Clostridium sp. DhR-2/LM-G01, Bacterial clone DCE25 and Bacterial clone DPHB06 were enriched in the methanogenic ASBR amended TCA.