Cyanobacteria could use sugars as carbon source and reductant to produce hydrogen by nitrogenase. However, oxygen is also produced during photosynthesis and it is an inhibitor of the enzyme nitrogenase. Filamentous cyanobacterium Anabaena sp. CH(3) could use sugars as substrate to produce molecular hydrogen anaerobically. The production activity was dependent on growth phases. It was found that the cells at sub-stage of late-log phase had better ability to produce hydrogen than at log phase. In such case, oxygen content was too low to be detected to inhibit hydrogen production. Among different kinds of sugar, fructose and glucose had the best performance for producing hydrogen. Hydrogen could be accumulated to 0.6 mmol (in 40 ml head space) in 100 h from 1000 ppm fructose. Increasing light intensities from 65 to 130 mu mol m(-2) s(-1) would enhance hydrogen production to 0.8 mmol. Under illumination of 130 mu mol m(-2) s(-1) and 2000 ppm fructose, 1.7 mmol of hydrogen could be accumulated. When fructose content was higher than 2000 ppm, cells could not produce more hydrogen at all. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.