Eicosapentaenoic acid (EPA) productivity from continuous cultures of the marine microalga Isochrysis galbana was studied, taking into account the irradiance on the reactor surface, that is, the photolimitation/photoinhibition regime to which the cells are exposed. Experiments were conducted under a wide variety of operating conditions. The dilution rate ranged from 0.005 h(-1) to 0.040 h(-1) at five external irradiances (820, 1620, 2050, 2450 and 3270 mu mol photons m(-2) s(-1)) covering photolimited to photoinhibited growth. Under these conditions, the specific growth rate (mu) was found to be the main factor influencing EPA content (ranging from 2.35% to 5.23% dryweight) and productivity (up to 0.88 mg l(-1) h(-1)). The fatty acid content was not significantly affected by the external irradiance, but was influenced by the state of growth of the microalga, depending on whether the light regime was photolimiting or photoinhibiting. It might be suggested that light should no longer be considered an isolated factor affecting EPA synthesis, but an indirect influence through the photolimitation/photoinhibition regime and growth rate. At a given dilution rate, EPA content and biomass concentration are lower under photoinhibiting external irradiances than those corresponding to photolimiting conditions, and consequently EPA productivity decays. Since the effect of photoinhibition is less marked at high biomass concentration, a strategy to optimize EPA productivity from microalgal cultures could consist of reducing the dilution rate when the external irradiance increases above the phoinhibition threshold.