The effect of different aeration rates and CO2 supply regimes on mass cultures of the marine microalga Dunaiella tertiolecta was studied. Eight aeration rates in the range 0-6.51 litres of air min-1 litre of culture-1 were tested. The results were compared to those obtained in non-aerated cultures into which pure CO2 was introduced and to cultures into which air enriched with CO2 was bubbled. The growth rate and final cellular density of D. tertiolecta in a sea-water-based medium increased with the aeration rate in the culture vessel. The maximal cellular density was 12.46 X 10(6) cells ml-1 under an air flow rate of 6.51 litres min-1 litre of culture-1, but evaporation and salinity increased sharply at this high aeration rate. The final cell density was proportional to the air flow rate and CO2 following the range (figures being litres of air min-1 litre of culture-1): 6.51 = 3.72 > 1.86 > CO2 = (0.93 + CO2) > 0.93> 0.46 > 0.23 > 0.11 > 0 (Mann-Whitney test p < 0.05). When D. tertiolecta was grown under a CO2 supply within the optimal pH levels but without aeration, the cultures reached a cell density of 7 X 10(6) cells ml-1. D. tertiolecta growth rate was inversely proportional to pH, the upper boundary for maximal growth rate being pH 9.2. Oxygen in the culture media produced by the photosynthetic activity of the microalgae did not inhibit growth. The dissolved CO2 concentration in seawater was the limiting factor for D. tertiolecta growth. At a pH value of 8.3, D. tertiolecta was not able to take up carbon in the form of carbonates dissolved in seawater.