The influence of the inoculum source and the acclimation strategy on the 4-chlorophenol (4CP) degradation in a sequencing batch reactor (SBR) was studied. Three different sources of inocula were obtained from the aeration tank of domestic, municipal and industrial wastewater treatment plants. The acclimation was performed using two strategies, the first one fixing the reaction time, independent of the removal efficiency (fixed time) and the second one fixing a removal efficiency of 90% as 4CP (variable time). The degradative activity was followed for each condition. Bacterial identification was carried out at the beginning and at the end of the experiments. Variable time strategy produced a microbial community with higher specific activity compared with those obtained for the fixed time strategy. The microbial activity was dependent of the origin of the inoculum. Each inoculum presented different specific activity to 4CP degradation. It was observed that the use of the fixed time strategy for the acclimation reduced the bacterial community diversity. (C) 2004 Elsevier Ltd. All rights reserved.