Two distinct full-scale upflow anaerobic sludge blanket (UASB) reactors, namely IUASB (143 m(3)) and TUASB (476 m(3)), treating alcohol (raki) distillery wastewaters were investigated in terms of performance, acetoclastic methanogenic activity and archaeal composition. The TUASB reactor was inoculated with the seed sludge taken from the IUASB reactor. An average of 90% COD removal efficiency at organic loading rates (OLRs) in a range of 6-11 kg COD/m(3) day was achieved in the IUASB reactor. However, 60-80% COD removal efficiency of the TUASB reactor was achieved at OLRs ranging 2.5-8.5 kg COD/m(3) day. Both UASB reactors showed almost the same acetoclastic methanogenic activity (350 and 376 ml CH4/g VSS day). Distribution of archaeal populations within the two UASB reactors was investigated by denaturing gradient gel electrophoresis analysis of PCR amplified ribosomal RNA gene fragments. Both UASB reactors have shown quite similar archaeal composition (weighted similarity of 83.1%). Predominant archaeal sequences in both reactors belonged to Methanobacterium formicicum and Methanosaeta soehngenii. (C) 2005 Elsevier Ltd. All rights reserved.