Highly unsaturated fatty acids (HUFAs), such as eicosapentaenoic acid and docosahexaenoic acid, are essential for normal growth and development and play an important role in the prevention and treatment of coronary artery disease, hypertension, arthritis and cancer. Enzymes that lengthen the carbon chain of polyunsaturated fatty acids are vital to biosynthesis of the highly unsaturated fatty acids, arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid from linoleic and a-linolenic acids. ELOVL5) is a member of the ELOVL family and is mainly responsible for 18 carbon-carbon chain extension. biosynthesis of HUFA, the full-length ELOVL5 cDNA from mirror carp was cloned using cDNA was 1 121 bp, with an open reading frame of 876 bp, encoding a protein of 291 amino acids containing a single histidine box, a canonical ER retention signal and several transmembrane regions, as seen in other fatty acid elongases. The protein shares 79.0%–93.1% sequence identity with other fish, and 69.0% identity with . The phylogenetic tree showed that it clustered closely with herbivorous and omnivorous freshwater fish. gene expression in various mirror carp tissues was determined using real-time quantitative PCR. Expression was highest in liver, followed by brain, and lowest in muscle. The fact that the ELOVL5 expression level was highest in brain and liver than in other tissues indicates that these are the main tissues for HUFA biosynthesis. We found the expression level lowest in muscle and this may be related to its in this tissue. These results will assist understanding of the biosynthesis of HUFA in common carp, and enable methods to be developed for enhancing its production. Enhanced HUFA production will accelerate the development and application of formulated feeds, and assist the development of aquiculture.