The possibility of hydrogen production by a hydrogenase impaired mutant strain of Anabaena variabilis in outdoor culture was studied. A. computer-controlled rooftop (outdoor) tubular photobioreactor (4.35 L) was assembled. H-2 production rates by A. variabilis PK84 grown in CO2 + air in the photobioreactor were measured together with other parameters such as temperature, irradiance, pH, dry biomass weight, and pO(2), and Chl a concentrations during summer months of 1998 and 1999. Efficiencies of light energy bioconversion to H, energy and energy accumulated in biomass were calculated. The influence of irradiance, temperature, and mode of cultivation on H-2 production and efficiency of light energy bioconversion were evaluated. The culture produced up to 1.1 L H-2 day(-1) PhBR-1. The efficiency of light energy to H-2 energy bioconversion on some days was 0.094%. However, the conditions for maximum H-2 photoproduction and for maximum efficiency of light energy to H-2 energy bioconversion were not the same. A. variabilis PK84 could produce hydrogen for prolonged periods (up to 40 days) without injection of fresh inoculum. During this period photobioreactor produced 24.5 L of H-2. Possibilities for increasing the efficiency of light energy conversion are discussed.