Recently, microalgae have gained a lot of attention because of their ability to produce fatty acids in their surrounding environments. The present paper describes the influence of organic carbon on the different fatty acid pools including esterified fatty acids, intracellular free fatty acids and extracellular free fatty acids in Ochromonas danica. It also throws light on the ability of O. danica to secrete free fatty acids in the growth medium under photoautotrophic and mixotrophic conditions. Biomass production of photoautotrophically grown O. danica was higher than that of mixotrophically grown, where a cellular biomass formation of 1.8 g L(-1) was observed under photoautotrophic condition which was about five folds higher than that under mixotrophic conditions. Contrary, the esterified fatty acid content reached up to 99 mg g(-1) CDW under photoautotrophic conditions at the late exponential phase, while during mixotrophic conditions a maximum of 212 mg g(-1) CDW was observed at the stationary phase. Furthermore, O. danica cells grown under mixotrophic conditions showed higher intracellular free fatty acid and extracellular free fatty acid contents (up to 51 and 20 mg g(-1) CDW, respectively) than cells grown under photoautotrophic conditions (up to 26 and 4 mg g(-1) CDW, respectively). The intra- and extracellular free fatty acids consisted of a high proportion of polyunsaturated fatty acids, mainly C18:2n-6, C18:3n-3 and C20:4n-6.