A factorial experimental design with two nutrient concentrations (2 and 4 mmol Nl(-1) in the form of NaNO3) and five rates of daily renewal of the cultures (10%, 20%, 30%, 40% and 50%) was carried out in cyclostat, light/dark-synchronized cultures of the marine microalga Dunaliella tertiolecta Butcher. Steady-state cellular density was a linear function inversely proportional to renewal rate. Maximal cellular productivity, 3 x 10(9) cells l(-1) day(-1), equivalent to 0.24 gl(-1) day(-1) dry weight and 0.17 gl(-1) day(-1) organic weight, was found with renewal rates of 20%-30% and 4 mmol Nl(-1), but maximal protein productivity, 0.066 gl(-1) day(-1), was obtained with a renewal rate of 40% for both nutrient concentrations. The protein content ranged between 30% and 70% of the organic fraction depending on the culture conditions. Carbohydrates were the only fraction accumulating in response to nutrient stress, ranging from 57% to 10% of the organic fraction, meanwhile the lipid content was increased by increase of nutrient availability. Under non-nitrogen-limited conditions the C:N ratio stabilized around 5.2-5.3 and the protein content of the organic fraction around 70%, but the cell nitrogen quota decreased under these conditions with increasing renewal rates, owing to the lower organic content of cells obtained with high growth rates. The high capacity for changing the biochemical composition, demonstrated for D. tertiolecta in the cyclostat system, has interesting implications for the management of continuous cultures of microalgae and its applications in biotechnological processes.