A novel continuously stirred anaerobic bioreactor (CSABR) seeded with silicone-immobilized sludge was developed for high-rate fermentative H2 production using sucrose as the limiting substrate. The CSABR system was operated at a hydraulic retention time (HRT) of 0.5–6 h and an influent sucrose concentration of 10–40 g COD/L. With a high feeding sucrose concentration (i.e., 30–40 g COD/L) and a short HRT (0.5 h), the CSABR reactor produced H2 more efficiently with the highest volumetric rate () of 15 L/h/L (i.e., 14.7 mol/d/L) and an optimal yield of ca. 3.5 mol H2/mol sucrose. The maximum value obtained from this work is much higher than any other values ever documented. Formation of self-flocculated granular sludge occurred during operation at a short HRT. The granule formation is thought to play a pivotal role in the dramatic enhancement of H2 production rate, because it led to more efficient biomass retention. A high biomass concentration of up to 35.4 g VSS/L was achieved even though the reactor was operated at an extremely low HRT (i.e., 0.5 h). In addition to gaining high biomass concentrations, formation of granular sludge also triggered a transition in bacterial community structure, resulting in a nearly twofold increase in the specific H2 production rate. According to denatured-gradient-gel-electrophoresis analysis, operations at a progressively decreasing HRT resulted in a decrease in bacterial population diversity. The culture with the best H2 production performance (at HRT = 0.5 h and sucrose concentration = 30 g COD/L) was eventually dominated by a presumably excellent H2-producing bacterial species identified as Clostridium pasteurianum. © 2005 Wiley Periodicals, Inc.