Anaerobe fermentation of sugar beet pressed pulp was investigated in pilot-scale digesters. Thermophilic adaptation of mesophilic culture was monitored using chemical analysis and metagenomic characterization of the sludge. Temperature adaptation was achieved by increasing the temperature gradually (2 degrees C day(-1)) and by greatly decreasing the OLR. During stable run, the OLR was increased gradually to 11.29 kg VS m(-3) d(-1) and biogas yield was 5% higher in the thermophilic reactor. VFA levels increased in the thermophilic reactor with increased OLR (acetic acid 646 mg L-1, propionic acid 596 mg L-1), then VFA decreased and the operation was manageable beside the relative high tVFA (1300-2000 mg L-1). The effect of thermophilic adaptation on the microbial communities was studied using a sequencing-based metagenomic approach. Connections between physico-chemical parameters and populations of bacteria and methanogen archaea were revealed. (C) 2014 Elsevier Ltd. All rights reserved.