The genetic background and activities of the enzymes involved in H<SUB>2</SUB> production were investigated from ten distinct H<SUB>2</SUB> producing cyanobacteria, revealed by a recent screening. All strains are N<SUB>2</SUB>-fixing, filamentous and heterocystous. Southern hybridization revealed that the tested strains possess the genes encoding the conventional nitrogenase (nifHDK1), and lack the alternative nitrogenases. The high H<SUB>2</SUB> production rate of these strains was shown not to be dependent on the presence of highly active nitrogenase or bidirectional hydrogenase enzymes. Moreover, most of the strains possessed a highly active uptake hydrogenase enzyme. We also examined the structure of the nif and hup operons encoding nitrogenase and uptake hydrogenase enzymes in the Calothrix 336/3 strain, the best H<SUB>2</SUB> producer in the screening. We concluded that the ability of the cyanobacteria to produce high levels of H<SUB>2</SUB> is not directly linked to the maximum capacities of the enzymes involved in H<SUB>2</SUB> production.