The subject invention relates to the identification of genes involved in the desaturation of polyunsaturated fatty acids at carbon 5 (i.e., ".DELTA.5-desaturase") and at carbon 6 (i.e., ".DELTA.6-desaturase") and to uses thereof. In particular, .DELTA.5-desaturase may be utilized, for example, in the conversion of dihomo-.gamma.-linolenic acid (DGLA) to arachidonic acid (AA) and in the conversion of 20:4n-3 to eicosapentaenoic acid (EPA). Delta-6 desaturase may be used, for example, in the conversion of linoleic (LA) to .gamma.-linolenic acid (GLA). AA or polyunsaturated fatty acids produced therefrom may be added to pharmaceutical compositions, nutritional compositions, animal feeds, as well as other products such as cosmetics.