Polyvinyl alcohol (PVA) gel beads cultivated in an upflow anaerobic sludge-blanket reactor treating synthetic corn steep liquor (CSL) wastewater were used to seed a lab-scale anaerobic fluidized-bed (AFB) reactor also treating CSL Under steady-state conditions in the AFB reactor, a COD removal efficiency in excess of 96% was achieved at an organic loading rate (OLR) of 25 g/l/d with a hydraulic retention time (HRT) of 10 h. Furthermore, a removal efficiency of 91% was achieved at an OLR of 27.5 g/l/d with an HRT of only 6 h. With a biomass attachment 1.02 g VSS/g PVA-gel beads, the biomass concentration in the AFB reactor was approximately 610 g/l, while for natural granules only 100 g/l is considered possible. Over the study period, the granular PVA gel turned from black to gray while the biomass attached on the outer surface of the beads also changed in composition. In addition, observations of the interior of the gel beads showed that colonization had progressed well into the core, indicating that effective substrate diffusion occurred within the PVA-gel matrix under the good substrate-microorganism contact conditions provided by the AFB reactor. (C) 2008, The Society for Biotechnology, Japan. All rights reserved.