Gamma-linolenic acid (GLA, gamma -C18:3) is an essential fatty acid that plays a vital role in biological structures and cellular functions. Based on available sequence information and using polymerase chain reaction (PCR) technique, we cloned from the fungus Mucor rouxii the entire coding sequence of a Delta (6)-desaturase enzyme, which is responsible for the transformation of linoleic acid into GLA. The deduced amino acid sequence of M. rouxii gene showed the highest homology with the plant Delta6-desaturase. It comprises the characteristics of membrane-bound desaturases, including histidine-rich motifs and hydrophobic regions. A cytochrome b(5)-like domain was observed at the N-terminus. In addition to three conserved histidine-rich motifs, we found an additional histidine-rich motif, HKHHSH, downstream of the cytochrome b(5)-like domain, which is not present in previously cloned Delta (6)-desaturase genes. Heterologous expression of the M. rouxii cDNA in Saccharomyces cerevisiae resulted in the synthesis and accumulation of GLA.