AbstractBoth Chlorella pyrenoidosa and Chlorella vulgaris grow equally well at 20°C aerated with ordinary air or mixtures of air with 5 or 12 per cent CO2 (5 klux continuous light). Whereas C. vulgaris relatively rapidly adapts to a higher CO2 tension, adaptation takes about 24 hours for C. pyrenoidosa.In Chlorella vulgaris pH in the range 3.6–7.6 has no apparent influence on the rate of photosynthesis in experiments having a duration of two hours. This is true both for algae grown aerated by ordinary air and for algae grown with a mixture of 5 per cent CO2 in air. The adaptation time must be short. In Chlorella pyrenoidosa the same is found for algae in ordinary air, whereas an influence of pH is seen in some experiments where the aeration was by 5 per cent CO2 in air.As is to be expected, the rate of photosynthesis in C. pyrenoidosa during the first two hours is very much influenced by the concentration of free CO2. The highest rate is found at the CO2 concentration at which the algae had been growing previously. The influence on the rate of photosynthesis in C. vulgaris is very much less, although in principle the same.The investigation of the corresponding influence on the rate of respiration is complicated by considerable variation from one series to another. In C. vulgaris this is particularly of importance. In C. pyrenoidosa, the highest rate of respiration is generally found at the CO2‐concentration at which the alga had been growing before the experiment.It seems probable that variations between similar series is due to the fact that the algae were grown in continuous light but with dilution with fresh culture medium when the optical density had reached a certain magnitude. Algae grown in this way are neither synchronized nor non‐synchronized.Our thanks are due to the Danish State Research Foundation for financial support.