The present invention provides a polypeptide having a wide substrate specificity and having a .omega.3 fatty acid desaturation activity, which makes efficiently unsaturated bond at .omega.3 position, and a polynucleotide coding for the same. By expressing the polypeptide in the organism, mass production of n-3 series PUFAs is enabled. Specifically, the polypeptide having a .omega.3 fatty acid desaturation activity and consisting of the amino acid sequence represented by SEQ ID NO: 1, a polynucleotide coding for a polypeptide having a .omega.3 fatty acid desaturation activity that consists of the base sequence represented by SEQ ID NO: 2 or SEQ ID NO: 3, and the like is useful for production of n-3 series fatty acids.