The aim of the present study was to investigate the effect of gradually increasing glucose concentrations (from 5.6 to 111 mmol L-1) on the fermentative H-2 production with and without bioaugmentation. A stirred tank reactor (STR) was operated at 70 degrees C and inoculated with a hyperthermophilic mixed culture or a hyperthermophilic mixed culture bioaugmented with Thermotoga neapolitana. With both the unaugmented (control) and augmented cultures, the H-2 production rate was improved when the initial glucose concentration was increased. In contrast, the highest H-2 yield (1.68 mol H-2 mol(-1) glucose consumed) was obtained with the augmented culture at the lowest glucose concentration of 5.6 mmol L-1 and was 37.5% higher than that obtained with the unaugmented culture at the same feed glucose concentration. Overall, H-2 production rates and yields were higher in the bioaugmented cultures than in the unaugmented cultures whatever the glucose concentration. Quantitative polymerase chain reaction targeting T. neapolitana hydA gene and MiSeq sequencing proved that Thermotoga was not only present in the augmented cultures but also the most abundant at the highest glucose concentrations. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.