This study investigated hydrogen fermentation from galactose in a fixed bed reactor (FBR) under a mesophilic temperature of 37 degrees C. The fixed bed reactor was packed with Lantec HD-Q-PAC material to support biomass growth and microbial assisted granule formation over the time course of the reactor operation. Hydraulic retention times (HRT) of 1.5-12 h over 79 days of bioreactor operation were assessed to determine the optimal hydrogen production efficiency of the system. The maximum hydrogen production rate and hydrogen yield of 65.5 L/L/d and 2.60 mol/mol hexoseadaed were achieved at a hydraulic retention time (HRT) of 2 h, which is higher than the reported maximum hydrogen production performance of reactors fed with galactose using other reactor configuration. The major volatile fatty acids formed were butyric and acetic acids. Microbial community analysis by quantitative real time polymerase chain reaction revealed that population changes greatly affected hydrogen production performance. The increase in the bacterial fraction of Lactobacillus spp. over 21.5% at a 1.5 h HRT led to increased lactic acid production up to 1865 mg/L, which deteriorated the reactor performances. Conversely, a bacterial fraction of Clostridium butyricum over 98% was observed when the H-2 production performance and organic acid distribution were recovered using a 2 h HRT. FBR would be a promising reactor configuration to achieve high hydrogen productivity while preventing the wash out of active biomass. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.