The performance of anaerobic digestion of abattoir wastewaters (AW) in an upflow anaerobic filter (UAF) was investigated under mesophilic (37 degrees C) and thermophilic (55 degrees C) conditions. The effects of increasing temperature on the performance of the UAF and on the dynamics of the microbial community of the anaerobic sludge were studied. The results showed that chemical oxygen demand (COD) removal efficiency of 90% was achieved for organic loading rates (OLRs) up to 4.5 g COD L-1 d(-1) in mesophilic conditions, while in thermophilic conditions, the highest OLRs of 9 g COD L-1 d(-1) led to the efficiency of 72%. The use of molecular and microbiological methods to recover microbial populations involved in this process showed that fermentative bacteria were the prominent members of the sludge microbial community. Three novel strains were identified as Macellibacteroides fermentans, Desulfotomaculum peckii and Defluviitalea saccharophila. (C) 2013 Elsevier Ltd. All rights reserved.