Comparison of measured primary production, by radioactive carbonate incorporation or oxygen production, with diel variations of oxygen, inorganic carbon and particulate carbon, shows that the net in situ production often exceeds the measured activity, especially in deeper marine ecosystems. In vitro incubations under fluctuating light conditions provide higher values of primary production at low light intensities, both with cultures and with natural populations. The same results are obtained with in situ incubations, by varying the depth of incubation. This method allows reevaluation of primary productivity at low light intensities and provides results in agreement with the in situ variations of oxygen, inorganic carbon and particulate carbon. It can explain why primary productivity in the ocean is higher than previously calculated