Chemical mutagenesis of the phototrophic green microalga Parietochloris incisa, producing high amounts of arachidonic acid (ARA), resulted in selection of a mutant deficient in ARA and rich in dihomo-gamma-linolenic acid (DGLA) and thus a dagger 5 desaturase defective. The mutagenesis produced a nonsense mutation in the a dagger 5 desaturase gene, resulting in alteration of the 62nd codon TGG into a stop codon. Thus, the polypeptide encoded by the mutant a dagger 5 desaturase gene is severely truncated and biochemically inactive, as was confirmed by heterologous expression in Saccharomyces cerevisiae. The mutation did not affect the oleogenic nature of the strain, and the total fatty acid content in the mutant biomass reached 39%, in comparison to 34% in the wild type, after 14 days of nitrogen starvation; biomass yields amounted to 5.1 and 3.6 g/l, respectively. While in the wild type, DGLA and ARA comprised about 1% and 58% of total fatty acids, respectively; the mutation annulled ARA almost totally but increased the DGLA proportion to 32% only with a corresponding increase in the proportion of oleic acid. Consequently, DGLA comprised 12.3% of dry weight, in comparison to 19.4% ARA in the wild type. The expression profiles of the genes coding enzymes involved in VLC-PUFA biosynthesis, a dagger 12, a dagger 6, a dagger 5 desaturases and a dagger 6 PUFA elongase, during nitrogen starvation, were compared. The transcript levels of all four genes, which were coordinately up-regulated in the wild type, appeared to be drastically reduced in the mutant, indicating their co-regulated transcription.