Eicosapentaenoic acid (EPA) volumetric productivity from an outdoor chemostat culture of Phaeodactylum tricornutum UTEX 640 in a 50-l tubular photobioreactor varies with dilution rate, reaching a maximum of 47.8 mg l(-1) d(-1) at D = 0.36 d(-1). Continuous culture at high dilution rates’ is proposed as the most adequate operating mode to maximize polyunsaturated fatty acid production.