The teleost warm temperature acclimation related 65 kDa protein (Wap65) is a plasma glycoprotein with the potential roles in heat adaptation, heme recycling, immune response and copper metabolism. It is most homologous to the mammalian hemopexin, which is the plasma transporter of heme. A full-length cDNA clone of the Wap65 gene, 1,534 bp in size, was isolated from the fish ayu (Plecoglossus altivelis). Its deduced amino acid sequence of 439 residues had 60.4-65.4% and 38.3-47.3% identical to fish Wap65-2-type and Wap65-1-type sequences, respectively. In phylogenetic analysis, aWap65 grouped tightly with those fish Wap65-2-type sequences. In healthy control fish, the highest mRNA signal for aWap65 was from the liver, moderately high in brain and gill, and but weaker in spleen, kidney, muscle, heart and intestine. In Listonella anguillarum-infected fish, aWap65 transcripts were significantly increased in liver, while no obvious changes in other tissues at 12 hpi. However, aWap65 transcripts were significantly increased in various tissues at 24 hpi when hemolysis developing, suggesting that aWap65 might be involved in the immune response of ayu.