In this study, the performance of 5.4 L hybrid upflow anaerobic sludge blanket (HUASB) reactor for treating poultry slaughterhouse wastewater under mesophilic conditions (29-35 degrees C), was investigated. After starting-up, the reactor was loaded up to an OLR of 19 kg COD/m(3) d and achieved varied TCOD and SCUD removal efficiencies of 70-86% and 80-92%, respectively. The biogas was varied between 1.1 and 5.2 m(3)/m(3) d with the maximum methane content of 72%. The maximum methane yield was 0.32 m(3)/kg COD(removed) at an OLR of 9.27 kg COD/m(3) d. Black matured granules of size between 2.5 and 5 mm were observed at the end of 225 d operation. RTD study showed the flow behavior was in mixed regime at the end of performance study. Step wise polynomial regression analysis was fitted well. Methanobacterium and Methanosaeta bacteria were dominant at the end of start-up whereas Methanosarcina, Cocci and rods were predominant at the end of performance studies. (C) 2011 Elsevier Ltd. All rights reserved.