- A strain improvement program was initiated based on mutagenesis with the goal of commercial production of eicosapentaenoic acid (EPA) from EPA- overproducing microalgal strains. Two rounds of mutation and selection were conducted using Phaeodactylum tricornutum Bohlin UTEX 640 as the parent strain. After the first round of mutagenesis, a putative mutant (provisionally labeled I14) was obtained. The EPA content (% of dry weight) of this mutant strain was 37% higher than that of the wild type. I14 was further mutated and another putative mutant (provisionally called II242) was isolated, the EPA content of which was 44% higher than that of the wild type. When cultured with aeration in I-L flasks, EPA content of the wild type and putative mutants 114 and II242 was 17.3 mg · g-1, 31.5 mg · g-1, and 38.6 mg · g-1 dry biomass, respectively. EPA productivity was 3.48 mg · L-1 · d-1, 4.01 mg · L-1 · d-1, and 4.98 mg · L-1 · d-1, respectively. These figures compare favorably with many other promising EPA- producing microorganisms and suggest that the use of a single methodology such as mutation and selection is a way to improve the polyunsaturated fatty acid content of microalgae and other microorganisms.