The present invention relates to fungal .DELTA.12 fatty acid desaturases that are able to catalyze the conversion of oleic acid to linoleic acid (LA; 18:2). Nucleic acid sequences encoding the desaturases, nucleic acid sequences which hybridize thereto, DNA constructs comprising the desaturase genes, and recombinant host microorganisms expressing increased levels of the desaturases are described. Methods of increasing production of specific .omega.-3 and .omega.-6 fatty acids by over-expression of the .DELTA.12 fatty acid desaturases are also described herein.