The distinctive features of cultivation of two strains of thermophilic cyanobacteria, Mastigocladus laminosus and Anabaena variabilis, in a pouring-off-filling-up type of photobioreactor are investigated. The dependence of the biomass growth rate and concentration doubling time on the carbon dioxide content in the injected air is studied and the culture medium composition is optimized. It is shown that tap water added with macro- and microelements, instead of analogous distilled water based nutrient medium, facilitates faster biomass growth. The minimum biomass doubling time is 0.9 day for Mastigocladus laminosus and 1.1 days for Anabaena variabilis when the carbon dioxide content in the injected air is 4 vol. %.