The present invention relates to a .DELTA.12 fatty acid desaturase able to catalyze the conversion of oleic acid to linoleic acid (LA; 18:2). Nucleic acid sequences encoding the desaturase, nucleic acid sequences that hybridize thereto, DNA constructs comprising the desaturase gene, and recombinant host microorganisms expressing increased levels of the desaturase are described. Methods of increasing production of specific .omega.-3 and/or .omega.-6 fatty acids are described by overexpression of the .DELTA.12 fatty acid desaturase or by disruption of the native gene.