Environmental as well as economic factors call for sustainable alternatives to the use of fossil fuels. Biofuels derived from arable crops are not cost-effective and the impact of these ‘first generation biofuels’ on food supply and price have raised ethical questions. Photosynthetic microorganisms offer an efficient means for biofuel production that is not associated with the current problems of land-based feedstock. We genetically modified S. elongatus in order to increase accumulation of glycogen, a raw material for bioethanol production.