The effect of temperature on the functionality of microbial community structure in a low temperature, anaerobic fixed-bed reactor was studied by decreasing the operating temperature from 18 degrees C to 5 degrees C. The reactor was productive within 20 days and produced stable methane content in biogas (above 77%) throughout the trial period. At 17 degrees C and 15 degrees C. chemical oxygen demand (COD) removal efficiency and biogas production of reactor were significantly reduced. These might be temperature thresholds when fixed-bed reactors are operated under low temperatures. The methanogen community composition was analyzed using 165 rRNA gene clone library screening and quantitative PCR. At low ambient temperatures. Methanomicrobiales were dominant methanogens, and they preferentially adhered to the carbon fiber carrier. The results indicated that 16S rRNA levels of Methanomicrobiales and Methanosaetaceae in adhering sludge were higher than in deposited sludge, and they all contributed to the efficient performance of the fixed-bed reactor at low operating temperatures. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.