The marine diatom Phaeodactylum tricornutum Bohlin is a potential source of the pharmaceutically valuable omega 3 polyunsaturated fatty acid eicosapentaenoic acid (EPA). The results of indoor continous growth of Phaeodactylum tricornutum Bohlin ar reported. The relationships between dilution rate (D), nitrate concentration and chemical composition were studied. Higher biomass and lipid productivities were obtained at low D values. EPA was found to be an intermediate metabolite and the best productivity (6 mg l(-1) day(-1)) was achieved for D values ranging from 0.32 to 0.50 day(-1). Under optimum conditions, 84 and 11%, respectively, of total recovered EPA were present in monogalactosyldiacylglycerol (MGDG) and in triacylglycerol (TG) moieties, respectively. Recorded EPA/AA and EPA/20.4 omega 3 ratios for all tested dilution rates were among the highest values ever reported, showing EPA purification to be easier to perform from this starting material than from many others commonly in use. (C) 1996 Elsevier Science Ltd.