The anaerobic biological treatment of raw sewage under low ambient temperatures was investigated in an up-flow anaerobic fixed bed (UAFB) reactor, over a 220-day trial period. The operating temperature was lowered stepwisely from 35 to 25.20 and 15 degrees C. Obvious accumulation of acetic acid was observed in the effluent as the temperature dropped, which almost tripled at 15 degrees C compared to that at 35 degrees C. An interesting psychrophilic-forward-shift trend of methane production rate following the decrease of temperature was discovered. The methane yield at each trial was 169, 201, 256 and 269 L-STP CH4/kg CODremoved. For each temperature trial, about 32.6%, 28.3%, 34.4% and 34.9% were removed in form of methane. Based on PCR-DGGE results, the band corresponded to Methanosphaera spp. only showed up on 15 degrees C profile. Analysis of both methane production and DGGE results indicated that psychro-tolerant microorganisms dominated the archaeal populations in the UAFB under low-temperature. (C) 2010 Elsevier Ltd. All rights reserved.