In this study, the H-2-producing activity of the sewage sludge microflora using xylose as the sole carbon substrate was investigated in three bioreactor systems, including a suspended continuously stirred tank reactor (CSTR), an immobilized-cell continuously stirred anaerobic bioreactor (IC-CSABR), and a powder activated carbon-assisted agitated granular sludge bed (AGSB). For suspended-culture CSTR operations, fermentative H-2 production was conducted at different hydraulic retention times (HRT=4 - 12 h). The H-2 production rate (HPR) was 183 mmol/L/d at HRT = 12 h, but the H-2 production rate and yield decreased significantly when the HRT was shortened to 4 h due primarily to the washout of cells. To improve the operational stability of short HRTs, silicone-immobilized cells (IC-CSABR system) and powder activated carbon carriers (AGSB system) were adopted for the reactor systems to either maintain stable biomass concentration in the reactor or enhance biomass content by stimulating sludge granulation. Both IC-CSABR and AGSB showed improved biomass retention while operating at a HRT of 4 h. In particular, the biomass content in the IC-CSABR system (HRT = 2 h) went up to 8.03 g of VSS/L, leading to a drastic enhancement in the H2 production rate (1.06 mol/L/d). Bacterial community analysis by denatured gradient gel electrophoresis (DGGE) indicates a transition in bacterial composition in the CSTR under different HRT conditions. Moreover, under the same HRT (4 h), the major bacterial populations in the AGSB and IC-CSABR reactors were very different from those observed in the CSTR, indicating that the performance of H-2 production seemed to be in close connection with the bacterial community structure. Several H-2-producing bacterial strains (e.g., Clostridium butyricum and Klebsiella pneumoniae) were also detected in the sludge samples by DGGE and 16S rDNA sequence matching, revealing the effectiveness of the H-2-producing sludge used in this study.