The stable and optimized operation of photobioreactors (PBRs) is the most challenging task in photofermentative biological hydrogen production. The carbon to nitrogen ratio (C/N) in the feed is a critical parameter that significantly influences microbial growth and hydrogen production. In this study, the effects of changing the C/N ratio to achieve stable biomass and continuous hydrogen production using fed-batch cultures of Rhodobacter capsulatus YO3 (uptake hydrogenase deleted, hup-) were investigated. The experiments were carried out in 8 L panel PBRs operated in indoor conditions under continuous illumination and controlled temperature. Culture media containing different acetate (40-80 mM) and glutamate (2-4 mM) concentrations were used to study the effects of changing the C/N ratio on biomass growth and hydrogen production. Stable biomass concentration of 0.40 g dry cell weight per liter culture (gDCW/L(c)) and maximum hydrogen productivity of 0.66 mmol hydrogen per liter culture per hour (mmol/L(c)/h) were achieved during fed-batch operation with media containing 40 mM acetate and 4 mM glutamate, C/N = 25, for a period of over 20 days. A study on the effect of biomass recycling on biomass growth and hydrogen production showed that the feedback of cells into the photobioreactor improved biomass stability during the fed-batch operation but decreased hydrogen productivity. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.