The photomixotrophic hydrogen production was investigated in sulfur deprived Chlamydomonas reinhardtii cultures. The cultures were exposed to continuous illumination of various light intensities in 27-day batches. Light intensity of 70 x 2 mu E m(-2) s(-1) was selected for hydrogen production. Subsequent experiments involving 27-day long light:dark cycles were conducted at the selected light intensity. The cycles consisted of hour divisions (h:h; 18:6, 14:10, 12:12) or minute divisions (min:min; 45:15, 35:25, 30:30). The results showed an adverse effect of the light:dark cycles on hydrogen production. All experiments, irrespective of the type of illumination indicated that cultures needed a lag phase for production and the highest hydrogen production was obtained during first 7-10 days of production reaching a peak in the first 5 days. (C) 2010 Elsevier Ltd. All rights reserved.