Cytidine deaminase (CDA, also designated CDD) is a zinc-dependent enzyme involved in the pyrimidine salvage pathways and becoming very important in anticancer and antiviral therapy. Here we report the identification and characterization of a CDA homologue in abalone, which we named ab-CDA. The analysis of the amino acids sequence revealed that the ab-CDA shares conserved signature motifs and belongs to homotetrameric class of CDA family. Real-time PCR analysis indicated that the ab-CDA was ubiquitously expressed in various tissues of abalone and relatively higher expressed in hemocyte. Significant up-regulation of ab-CDA was also observed after LPS or Poly I: C challenge. The biological activity of ab-CDA was identified by spectrophotometry analysis and the intracellular localization displayed that ab-CDA was largely concentrated in the cytoplasm and partially in the nuclei. These results strongly suggest that ab-CDA is a CDA homologue and it is involved in the immune response of gastropod abalone.