The alternative methods should be used to treat sewage sludge and municipal solid wastes which are mostly landfilled in Turkey. Therefore co-composting of these waste streams is a suitable disposal method yielding a useful product. The aim of this study was to investigate the microbial community during the field scale co-composting of sewage sludge and organic municipal solid wastes with addition of bulking agents. Aerated static pile of approximately 26 m(3) was used for composting process during 56 days. Investigations of diversity dynamics depending on the temperature were determined by denaturing gradient gel electrophoresis and sequencing of bacterial 16SrDNA-PCR products. The variations of physicochemical parameters and biodegradability during the process were also monitored. The results showed that most of microbial group's role in composting process was temperature dependent and composting was designated by its characteristic thermal profile.