Non-axenic operation of a 400 L trickle bed reactor inoculated with the thermophile Caldicellulosiruptor saccharolyticus, yielded 2.8 mol H-2/mol hexose converted. The reactor was fed with a complex medium with sucrose as the main substrate, continuously flushed with nitrogen gas, and operated at 73 degrees C. The volumetric productivity was 22 mmol H-2/(L. filterbed h). Acetic acid and lactic acid were the main by-products in the liquid phase. Production of lactic acid occurred when hydrogen partial pressure was elevated above 2% and during suboptimal fermentation condition, that also resulted in the presence of mono- and disaccharides ill the effluent. Methane production was negligible. The microbial community was analyzed at two different time points during operation. Initially, other species related to members of the genera Thermoa naerobacterium and Caldicellulosiruptor were present in the reactor. However, these were out-competed by C. saccharolyticus during a period when sucrose was completely used and no saccharides were discharged with the effluent. In general, the use of pure cultures ill non-sterile industrial applications is known to be less useful because of contamination, However, our results show that the applied fermentation conditions resulted in it culture of it single dominant organism with excellent hydrogen production characteristics.