In this study, the long-term stability of biomass and hydrogen production on acetate by Rhodobacter capsulatus YO3 (hup(-)) was investigated. The experiments were performed in fed-batch panel photobioreactors operated under the natural sunlight in Ankara, Turkey. They were carried out between October and December in order to resemble low temperature and low light intensity and between July and August in order to resemble high temperature and high light intensity. At high temperature and high light intensity, stable biomass concentration of 0.68 g dry cell weight per liter culture (gdcw/L(c)) was obtained with hydrogen productivity of 0.51 mmol H(2)/L(c)/h and molar hydrogen yield of 53% for a period of 60 days. At low temperature and low light intensity hydrogen productivity of 0.30 mmol H(2)/L(c)/h and yield of 44% were obtained, while biomass decreased from 0.65 gdcw/L(c) to 0.31 gdcw/L(c) over 20 days. The daily yield factor (mol H(2)/gdcw) was shown to be related to the daily total global solar radiation (Wh/m(2)). Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.