We identified a cDNA expressed sequence tag from an animal (the nematode worm Caenorhabditis elegans) that showed weak similarity to a higher-plant microsomal Delta6-desaturase. A full-length cDNA clone was isolated and expressed in the yeast Saccharomyces cerevisiae. This demonstrated that the protein encoded by the C. elegans cDNA was that of a fatty acid Delta6-desaturase, as determined by the accumulation of gamma-linolenic acid. The C. elegans Delta6-desaturase contained an N-terminalcytochrome b5 domain, indicating that it had a similar structure to that of the higher-plant Delta6-desaturase. The C. elegans Delta6-desaturase mapped to cosmid W08D2, a region of chromosome III. This is the first example of a Delta6-desaturase isolated from an animal and also the first example of an animal desaturase containing a cytochrome b5 domain.