An outdoor tubular photobioreactor, based on an external-loop airlift, has been used to cultivate the diatom Phaeodactylum tricornutum UTEX 640, rich in icosapentaenoic acid (EPA). The system was operated in batch mode. The specific growth rate in the exponential phase, averaged over the whole day, was 0.254 day(-1). A 25 gl(-1) maximum biomass concentration was obtained by the end of the culture period. The dissolved oxygen generated never rose over 210% of air saturation nor was there impediment to biomass generation or inhibition of photosynthesis. Furthermore, around 2 x 10(17) quanta cm(-2) s(-1) light intensity seemed to saturate the photosynthetic activity. Biomass losses during the night could be reduced to less than 5% by maintaining the culture at 16 degrees C at night. The maximum concentration and productivity of EPA were 423 mgl(-1) and 13 mgl(-1)day(-1), respectively, at the end of the culture, although a 21-mgl(-1)day(-1) net EPA generation rate was obtained at the linear growth phase.