Chlamydomonas reinhardtii is a green alga that can use light energy to produce hydrogen from water under anaerobic conditions. This work reports the enhancement of hydrogen production by controlling the light intensity in sulfur-deprived anaerobic C. reinhardtii cultures. The overall hydrogen production was dependent on light intensity in the range of 60-200 mu E m(-2) s(-1). Maximum hydrogen production was obtained at a light intensity of 200 mu E m(-2) s(-1) as a result of the rapid initiation of hydrogen production and the greatest increase of chlorophyll during the initial 24h after sulfur deprivation. However, the hydrogen production was inhibited at an intensity of 300 mu E m(-2) s(-1) of light owing to photosystem II photodamage by excess light. The maximum hydrogen production and the maximum specific production rate of hydrogen were 225 ml H-2 l(-1) culture and 2.01 ml H-2 g(-1) cells h(-1), respectively. Thus, hydrogen production by sulfur-deprived C. reinhardtii cultures can be maximized by controlling the light intensity at levels below saturation. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.