Cyanobacteria are the only bacteria capable of performing oxygenic photosynthesis in which they harness solar energy and convert it into chemical energy stored in carbohydrates. Under specific conditions, cyanobacteria can use solar energy to produce also molecular hydrogen. Biodiversity among cyanobacteria for H(2) production has not been efficiently studied. Here we report the screening of 400 cyanobacterial strains isolated from the Baltic Sea and Finnish lakes for efficient H(2) producers. Approximately 50% of these strains produced detectable amounts of H(2). Ten strains produced similar or Lip to 4 times as much of H(2) as the hydrogenase mutants of Anabaena PCC 7120 and Nostoc punctiforme ATCC 29133 specifically engineered in different laboratories to produce higher amounts of H(2). All ten H(2) producers are N(2)-fixing filamentous, heterocystous strains, seven of them are benthic and three are planktonic strains. Different culturing parameters, such as light intensity, cell density, pH and temperature had a pronounced effect on the H(2) production rates of the two good H(2) producers, Calothrix 336/3 and XPORK 5E strains. Notably, the culture conditions for optimal H(2) production varied between different cyanobacterial strains. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.