The toxicity effects of methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA) and formaldehyde (FA) on indigenous microbial community in forest soil were studied. MTBE, TBA and FA with different concentrations were added into microcosms containing forest soil samples. After 10 and 30 days, total viable cell number and dehydrogenase activity in the microcosms were evaluated. Bacterial communities in the microcosms were also analyzed using a denaturing gradient gel electrophoresis (DGGE). Dehydrogenase activity and total viable cell number were decreased according to the increase of MTBE, TBA and FA concentrations (P<0.05). FA toxicity was the the highest, and TBA toxicity was the lowest. The results of principal component analysis using DGGE fingerprints showed that the microbial communities contaminated with MTBE, TBA and FA were grouped by exposure time, but not by exposure concentration. Dominant species in the microcosms were as follows: gamma-proteobacteria, Chloroflexi, Actinobacteria, Clostridia and Bacilli f or M TBE, TBA a nd FA. The toxicity effects of methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA) and formaldehyde (FA) on indigenous microbial community in forest soil were studied. MTBE, TBA and FA with different concentrations were added into microcosms containing forest soil samples. After 10 and 30 days, total viable cell number and dehydrogenase activity in the microcosms were evaluated. Bacterial communities in the microcosms were also analyzed using a denaturing gradient gel electrophoresis (DGGE). Dehydrogenase activity and total viable cell number were decreased according to the increase of MTBE, TBA and FA concentrations (P<0.05). FA toxicity was the the highest, and TBA toxicity was the lowest. The results of principal component analysis using DGGE fingerprints showed that the microbial communities contaminated with MTBE, TBA and FA were grouped by exposure time, but not by exposure concentration. Dominant species in the microcosms were as follows: gamma-proteobacteria, Chloroflexi, Actinobacteria, Clostridia and Bacilli f or M TBE, TBA a nd FA.