Rates of respiratory oxygen uptake were measured in darkened samples of green microalgae (Coelastrum sphaericum, Scenedesmus falcatus) growing in raceway ponds under field conditions and indoors. The rates measured over 6–26h varied between 1 and 1.5 and 8.25 μl O2 mg−1 h−1 and depended on incubation, temperature, time in the dark, and on the temperature and irradiance at which the algae were cultivated before. Q10 values ranged from 1.43 to 1.68. Arrhenius relationships described the influence of temperature upon respiration rates below the optimum temperature, even after many hours of incubation in the dark. Respiration rates were lower after growth at optimal temperature than after cultivation at other temperatures. The light history also influenced the rates, being high when the algae were darkened after exposure to high irradiance and lower after weaker pre-illumination. For the algae under study the overall loss during 12h of darkness was estimated to be 2–10% of the biomass prior to darkening. Higher losses are expected for natural conditions and stronger irradiances.