It is well documented that culture conditions affect the fatty acid content of microalgae. We report in this study the fatty acid profiles and n - 3 HUFA productivity of T-ISO, a popular haptophyte in the aquaculture industry, cultured under three photoperiods (24:0, 16:08 and 12:12 h L:D) combined with three photon flux densities (PFD: 120, 220 and 460 Âµmol photon m-2 s-1), at 25 Â°C. Sampling took place in both the exponential and post-exponential (light-limited) phase. In general, fatty acid proportions were effected by a strong interaction of L:DÃ PFD resulting in metabolic changes difficult to be modelled. At the 12:12 and 24:0 h L:D the fatty acid pattern can be summarised as PUFA>SAFA>MUFA, while at 16:08 h L:D as SAFA>PUFA>MUFA reflecting a differential acclimation of the strain under light-dark cycles. At the 12:12 h L:D the PUFA content of biomass was significantly higher than at the other photocycles. PUFA content differences were located in the n - 3 fraction with the n - 6 content being rather constant. The n - 3/n - 6 and DHA/EPA ratios under 24:0 h or 12:12 h L:D were optimal according to the literature for fish and shellfish nutrition requirements. In contrast, the 16:08 h L:D regimes, especially at low PFD, produced inadequate ratios. The production of n - 3 HUFA in T-ISO is essentially influenced by the total photon flux available per day in a similar manner with growth. The capacity of the strain for storing lipid is limited under the conditions tested; consequently, the fatty acid content follows the biomass yield and productivity pattern. Hence, in the context of aquaculture a light regime of 12:12 h L:D and a PFD within the photolimitation-photoinhibition range offers advantages for the culture of T-ISO. If the high investment could be substantiated, continuous cultures under 24:0 h L:D at the same PFD range could serve as an optimisation basis using advanced photobioreactors.