Hydrogen (H-2) photoproduction and carbon dioxide (CO2) uptake by the cyanobacterium Anabaena variabilis was measured under partial vacuum in batch culture and in a photobioreactor. Increasing CO2 concentrations in the gas phase led to decreased H-2 photoproduction and increased CO2 uptake by the cells, which was accompanied by O-2 evolution. A laboratory-scale hollow-fiber photobioreactor was constructed for the production of H-2 and uptake of CO2 by the immobilized cyanobacteria. A two-phase H-2 production-CO2 uptake system is suggested as being feasible for practical demonstration. In the CO2 uptake phase the cyanobacterial cells consume CO2 at rates up to 150 to 170 ml CO2 g(-1) dry wt h(-1) and establish conditions for subsequent H-2 photoproduction. In the H-2 production phase the cyanobacteria evolve H-2 at rates up to 20 ml H-2 g(-1) dry wt h(-1). The photobioreactor was run continuously for over 1 year.