The present work reports on the influence of feeding strategy on the stability and performance of a stirred anaerobic sequencing fed-batch reactor containing biomass immobilized on polyurethane foam. The reactor treated low-strength wastewater and was operated at 30 C with an agitation rate of 200 rpm. A 180-min cycle was used to treat approximately 0.51 of synthetic substrate with a chemical oxygen demand concentration of nearly 500 mg/l. The reactor was operated in batch mode with a 3-min feeding step and in constant rate fed-batch mode with feeding steps of 30, 60 and 180 min. During batch operation, the system attained stability and had a removal efficiency of 86% based on non-filtered substrate concentration. However, during fed-batch operation stability and efficiency were impaired and formation of suspended material was identified. Stability was achieved only for the 30-min feeding step. The poor performance and instability observed in the fed-batch experiments were credited to the formation of considerable quantities of extracellular polymers, which impeded contact between substrate and biomass with consequent negative effect on the mass transfer fluxes. The biopolymer formation was very likely a result of the fed-batch operational mode, in which part of the bioparticles were deprived of contact with the liquid medium for a relatively long period of time. (C) 2003 Elsevier Ltd. All rights reserved.