The microbial community in the sludge blanket of a bench scale upflow anaerobic sludge blanket (UASB) reactor with partial recirculation of the effluent, treating wastewater from an unbleached pulp plant. was evaluated by means of the small subunit (SSU) rRNA gene amplified by polymerase chain reaction (PCR) and separated using the denaturing gradient gel electrophoresis (DGGE) technique. The average COD removal efficiency was 80-86% without recirculation and 75-78% with recirculation. Partial recirculation of the effluent did not alter the predominant population in the reactor under the conditions studied. A microscopic analysis revealed the presence of a variety of cellular morphologies. The DGGE results revealed that the structure of the microbial community was found to be complex and most of the population was similar, persisting throughout the experimental period. The microbial community of the Domain Bacteria was more sensitive to operational changes than the Domain Archaea. (C) 2005 Elsevier Ltd. All rights reserved.