Arachidonic acid (C20:4 Δ5,8,11,14)) is a polyunsaturated fatty acid synthesized by the ?-fatty acid desaturation of di-homo-?-linolenic acid (C20:3 Δ8,11,14)). In mammals, it is known to be a precursor of the prostaglandins and the leukotrienes but it is also accumulated by the filamentous fungus Mortierella alpina. We have isolated a cDNA encoding the Δ5-fatty acid desaturase from M. alpina via a polymerase chain reaction- based strategy using primers designed to the conserved histidine box regions of microsomaL desaturases, and confirmed its function by expression in the yeast Saccharomyces cerevisiae. Analysis of the lipids from the transformed yeast demonstrated the accumulation of arachidonic acid. The M. alpina Δ5- desaturase is the first example of a cloned Δ5-desaturase, and differs from other fungal desaturases previously characterized by the presence of an N- terminal domain related to cytochrome b5.