Photoautotrophic fatty acid production of a highly CO(2)-tolerant green alga Chlorococcum littorale was investigated in the presence of inorganic carbon and nitrate at 295 K and a light intensity of 170 micromol-photon m(-2) s(-1). CO(2) concentration in the bubbling gas was adjusted by mixing pure gas components of CO(2) and N(2) to avoid photorespiration and beta-oxidation of fatty acids under O(2) atmosphere conditions. Fatty acid content was almost constant for the CO(2) concentrations ranging from 5% to 50% under nitrate-rich conditions corresponding to the logarithmic growth phase. After nitrate depletion, the content drastically increased with a decrease in CO(2) concentration. HCO(3)(-)/CO(2) ratio in the culture media was found to be a controlling factor for fatty acid production after the nitrate limitation phase. For a CO(2) concentration of 5%, the fatty acid content was ca. 34 wt.% at maximum, which is comparable with other land plant seed oils.