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Soil core samples were obtained from a trichloroethylene (TCE)-contaminated aquifer before and after the start of methane biostimulation. DNA was extracted directly from the soil samples, and denaturing gradient gel electrophoresis (DGGE) was used to analyse bacterial 16S ribosomal DNA fragments that were PCR amplified from these DNA samples. This analysis consistently detected two phylotypes in the methane-injected samples. These phylotypes were closely related to Methylobacter and Methylomonas, both belonging to type I methanotrophs. A competitive DGGE analysis using Methylosinus trichosporium OB3b cells as an internal quantitative standard showed that these populations accounted for 108−109 cells g−1 soil. These results showed that type I methanotrophs formed a significant proportion of the bacterial community during methane biostimulation. The implications of this finding for TCE bioremediation were discussed.