- Interest in the polyunsaturated fatty acid eicosapentaenoic acid (EPA) as a therapeutic agent is steadily increasing. The microalga Monodus subterraneus produces EPA, which is concentrated mainly in the galactolipid fraction, as its major fatty acid. Nitrogen starvation increased the fatty acid content but reduced the proportion and content of EPA to 19.5%(of fatty acids) and 1.8%(of dry weight), respectively. Cultivation under low light intensity or high biomass concentration enhanced the proportion of EPA up to 36.7% of fatty acids and the content to 4.4% of dry weight. Maximal EPA productivity of 25.7 mg· L− 1· d− 1 was obtained at the biomass concentration that resulted in the highest biomass productivity. M. subterraneus is thus one of the most promising candidates for phototrophic production of EPA.