The effect of different aeration rates, in the range 0-6.51 l of air min(-1) l of culture(-1), and CO2 supply on the biochemical composition of mass cultures of the marine microalga Dunaliella tertiolecta was studied The biochemical composition of D, tertiolecta was strongly affected by the aeration rate. There was a negative correlation between stationary-phase protein cellular content and air flow. Carbohydrate cellular content also decreased with aeration rate, a minimum being reached with 0.93 l of air min(-1) l of culture (-1) Maximum carbohydrate per volume unit was achieved with maximum aeration as the increase of carbohydrates per mi was directly proportional to airflow and therefore to CO2 availability. Maximum protein per mi was achieved with 1.86 l of air min(-1) l of culture(-1), keeping stable with higher airflows. The cultures supplied with CO2 showed carbohydrate and protein concentrations similar to the cultures with 1.86 l of air min(-1) l of culture(-1), indicating a correlation between available CO2 and not only carbon but also nitrogen metabolism. Different factors seemed to limit cell division and nitrogen metabolism as maximum nitrogen transformation rate was achieved with an air flow of 1.86 l of air min(-1) l of culture(-1), lower than the 3.72 l of air min(-1) l of culture(-1) needed for maximum cell density.