The subject invention relates to the identification of genes involved in the desaturation of polyunsaturated fatty acids at carbon 4 (i.e., ".DELTA.4-desaturase"). In particular, .DELTA.4-desaturase may be utilized, for example, in the conversion of adrenic acid to .omega.6-docosapentaenoic acid and in the conversion of .omega.3-docosapentaenoic acid to docosahexaenoic acid. The polyunsaturated fatty acids produced by use of the enzyme may be added to pharmaceutical compositions, nutritional compositions, animal feeds, as well as other products such as cosmetics.