Coalbed methane (CBM) exploitation and CO2 sequestration are gaining global interest for which stimulating indigenous microorganisms to "solubilize" coal and to convert CO2 to methane can be one of the most important approaches. However, the indigenous microbial community of coal environments is still far from clear. For the first time in China, microbial communities in two coalfields along the Ordos Basin with different coalification levels were studied using 16S rRNA gene clone analysis. Phylogenetic analyses of 16S rRNA gene sequences revealed unexpectedly diverse microbial communities in the Ordos Basin, as compared to other coalbeds worldwide. Bacteria classified in the phyla Proteobacteria and Actinobacteria were found as dominant lineages in the low-ranking coal from 38 to 40 m depth, while those in the phyla Proteobacteria, Actinobacteria, Bacteroidetes, Planctomycetes and Acidobacteria were dominant in the high-ranking coal from 651 to 652 m depth. In addition, the bacterial community was more diverse than those in coals reported so far and the existence of an intact methanogenic microbial community was revealed in low-ranking coal. In contrast, the bacterial diversity decreased remarkably in the presence of sulfate reducing bacteria with no methanogens detected in coal with high coalification levels. (C) 2011 Elsevier RV. All rights reserved.