In order to determine the effects of the deletion of hydrogenase genes on nitrogenase-based photobiological H-2 productivity by heterocystous N-2-fixing cyanobacteria. we have constructed three hydrogenase mutants from Anabaena sp. PCC 7120: hupL(-) (deficient in the uptake hydrogenase), hoxH(-) (deficient in the bidirectional hydrogenase), and hupL(-/)hoxH(-) (deficient in both genes). The hupL(-) mutant produced H-2 at a rate four to seven times that of the wild-type under optimal conditions. The hoxH(-) Mutant produced significantly lower amounts of H-2 and had slightly lower nitrogenase activity than wildtype. H-2 production by the hupL(-)/hoxH(-) mutant was slightly lower than, but almost equal to, that of the hupL(-) mutant. The efficiency of light energy conversion to H-2 by the hupL(-) mutant at its highest H-2 production stage was 1.2% at an actinic visible light intensity of 10 W/m(2) (PAR) under argon atmosphere. These results indicate that deletion of the hupL gene could be employed as a source for further improvement of H-2 production in a nitrogenase-based photobiological H-2 production system.