An isolate of Isochrysis galbana rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) has been grown as a chemostat culture at 20 degrees C and pH 8.00 controlled by CO2 injection. From a low dilution rate (D) of 0.0024 h(-1) to 0.0377 h(-1), close to maximum growth, a decrease in EPA content from 5.21% dry weight (d.w.) to 2.80% d.w. was observed, although the percentage of EPA in the total fatty acids increased. Lipids were fractionated, EPA being the major fatty acid found in the glycolipid fraction, whereas in the neutral lipid fraction were mainly palmitic and palmitoleic acids. At the same time, the biomass concentration also decreased from 1015 mg.l(-1) to 202 mg.l(-1) over the range of Ds mentioned. Nonetheless, EPA. productivity had a maximum value of 15.26 mg.l(-1) day(-1) at D = 0.0208 h(-1).