The present invention relates to fatty acid desaturases and elongases able to catalyze the conversion of linoleic acid (LA) to .gamma.-linolenic acid (GLA); .alpha.-linoleic acid (ALA) to stearidonic acid (STA); GLA to dihomo-.gamma.-linoleic acid (DGLA); STA to eicosatetraenoic acid (ETA); DGLA to ETA; eicosapentaenoic acid (EPA) to docosapentaenoic acid (DPA); and arachidonic acid (ARA) to EPA. Nucleic acid sequences encoding codon-optimized desaturases and elongases, nucleic acid sequences which hybridize thereto, DNA constructs comprising the codon-optimized desaturase or elongases, and recombinant host microorganisms expressing increased levels of desaturase or elongase are described.