The present invention relates to a polynucleotide encoding an omega 3 (.omega.-3) desaturase from Pythium irregulare with specificity to long chain polyunsaturated omega 6 (.omega.-6) fatty acids as well as a vector containing said polynucleotide, and a host cell containing the vector or the polynucleotide. Moreover, the present invention pertains to a polypeptide encoded by the said polynucleotide, antibodies against the polypeptide as well as a method for the manufacture of the polypeptide. Further, encompassed by the present invention are transgenic non-human organisms. Finally, the present invention relates to methods for the manufacture of compounds and oil- fatty acid- or lipid-containing compositions.