This paper describes the continuous biohydrogen production in a mesophilic fixed-bed reactor using anaerobic digester sludge as an inoculum. Hydraulic retention times (HRTs) were decreased stepwise from 12 to 1.5 h, while 15 g/L of glucose was used as the model substrate. The peak hydrogen production performance was found at 1.5 h HRT with the hydrogen yield (HY) of 2.3 mol H-2/mol glucoseaadea and the hydrogen production rate (HPR) of 78 L H-2/L-d. Butyrate and acetate were the major soluble metabolic products released during the fermentation. Quantitative polymerase chain reaction (qPCR) and scanning electron microscopy analyses implied that Clostridium butyricum was dominant in the mixed culture fermentation in all the examined HRTs. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.