We have analyzed the filamentous cyanobacterium Anabaena PCC 7120 (wildtype) containing one nitrogenase, one uptake hydrogenase and one bidirectional hydrogenase and its hydrogen uptake deficient mutant AMC 414 for their H-2 production capacities. Anabaena PCC 7120 and AMC 414 had similar growth rates in turbidostat mode with increased growth rates at higher light intensity. Rates Of C2H2 reduction were similar for both strains. In contrast to the wildtype, AMC 414 produced H-2 in a PhotoBioReactor (PhBR) using air as the lifting gas. The rate of H-2 production increased with light intensity and was not even saturated at 456 muE m(-2) s(-1). H-2 production increased significantly when replacing the air with argon. The maximal H-2 production during outdoor conditions was recorded using AMC 414 with a peak at 14.9 ml H-2 h(-1) l(-1). Despite the relatively high production, maximal efficiency of solar energy to H-2 conversion was only 0.042%. A molecular method was developed to analyze the relative abundancies of weight and mutant in competition experiments in the PhBR. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.