We constructed three hydrogenase mutants from Anabaena sp. PCC 7120: DeltahupL (deficient in uptake hydrogenase), DeltahoxH (deficient in bidirectional hydrogenase), and DeltahupL/DeltahoxH (deficient in both genes), and showed that the DeltahupL and DeltahupL/DeltahoxH produced H-2 at a rate 4-7 times that of wild-type under optimal conditions (Appl. Microbiol. Biotechnol. 58 (2002) 618). We have studied H-2 producing activity of Deltahup in more detail. H-2 producing activity of Deltahup cells was moderately improved in older cultures when 1% CO2 was added to the bubbling air. The efficiency of light energy conversion to H-2 by the DeltahupL mutant at its highest H-2 production stage was 1.0-1.6% at an actinic visible light intensity of lower than 50 W/m(2) under argon atmosphere, and the activity lasted for at least 35 min. At 250 W/m(2), H-2 producing activity gradually decreased with illumination time. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.