The present invention relates to .DELTA.17 desaturases, which have the ability to convert .omega.-6 fatty acids into their .omega.-3 counterparts (i.e., conversion of arachidonic acid [20:4, ARA] to eicosapentaenoic acid [20:5, EPA]). Isolated nucleic acid fragments and recombinant constructs comprising such fragments encoding .DELTA.17 desaturases along with a method of making long-chain polyunsaturated fatty acids (PUFAs) using these .DELTA.17 desaturases in oleaginous yeast are disclosed.