We isolated a homolog of cathepsin D from the cDNA library of the olive flounder kidney. The olive flounder cathepsin D transcript consisted of 1,733 bp that encoded a polypeptide of 396 amino acids. The overall similarity between olive flounder cathepsin D and other cathepsin Ds was very high, with the highest amino acid sequence identity to barramundi perch (89%). RT-PCR revealed that cathepsin D was expressed in almost all tissues, with high expression in the liver, intestine, kidney, skin, and spleen. The accumulation of cathepsin D mRNA after bacterial infection, as determined by RT-PCR, was constitutive and increased greatly after bacterial infection.