This paper illustrates indoor and outdoor results of hydrogen photoproduction by Rhodopseudomonas palustris (strain 42OL). Three cylindrical photobio-  reactors of different internal diameters (PBR₁ = 7.6 cm; PBR₂ = 9.6 cm and PBR₃ = 13.0 cm) were operated indoors using an artificial irradiance of 483 W m^-2. An underwater tubular photobioreactor (UwTP) of 4.85 cm internal diameter was operated outdoors, in autumn. We demonstrated that the hydrogen photoevolution rate is greatly affected by the diameter of photobioreactors. An inverse relationship links the hydrogen production rate to the diameter of the reactor. The highest hydrogen photoevolution rate of 15.150 ± 0.597 ml l_culture^-1h^-1 (average maximum value calculated on four consecutive days) was obtained in PBR₁. The lengthiest period (528 hours) of hydrogen photoevolution was achieved in this photobioreactor as well. During outdoor investigation, the maximum hydrogen production rate was 4.16 l UwTP^-1d^-1; this value corresponds to 9.81 ml l_culture^-1h^-1.