The conversion of solar energy into hydrogen represents a highly attractive strategy for the production of renewable energies. Photosynthetic microorganisms have the ability to produce H-2 from sunlight but several obstacles must be overcome before obtaining a sustainable and efficient H-2 production system. Cyanobacteria harbor [NiFe] hydrogenases required for the consumption of H-2. As a result, their H-2 production rates are low, which makes them not suitable for a high yield production. On the other hand, [FeFe] enzymes originating from anaerobic organisms such as Clostridium exhibit much higher H-2 production activities, but their sensitivity to O-2 inhibition impairs their use in photosynthetic organisms. To reach such a goal, it is therefore important to protect the hydrogenase from O-2. The diazotrophic filamentous cyanobacteria protect their nitrogenases from O-2 by differentiating micro-oxic cells called heterocysts. Producing [FeFe] hydrogenase in the heterocyst is an attractive strategy to take advantage of their potential in a photosynthetic microorganism. Here, we present a biological engineering approach for producing an active [FeFe] hydrogenase (HydA) from Clostridium acetobutylicum in the heterocysts of the filamentous cyanobacterium Nostoc PCC7120. To further decrease the O-2 amount inside the heterocyst, the GlbN cyanoglobin from Nostoc commune was coproduced with HydA in the heterocyst. The engineered strain produced 400 mu mol-H-2 per mg Chlorophyll a, which represents 20-fold the amount produced by the wild type strain. This result is a clear demonstration that it is possible to associate oxygenic photosynthesis with H-2 production by an O-2-sensitive hydrogenase.