Activation-induced cytidine deaminase (AID) is essential for class switch recombination, somatic hypermutation, and gene conversion of immunoglobulin gene. In the present study, canine AID cDNA was cloned from the lymph node of a healthy dog by RT-PCR with rapid amplification of cDNA ends (RACE) method. The canine AID cDNA was 1,377 bp in length, and contained the entire open reading frame encoding 198 amino acids which had 94.9%, 94.4%, and 89.9% homology with human, mouse, and chicken homologues, respectively. Canine AID mRNA was expressed in thymus, lung, spleen, kidney, small intestine, lymph node, and tonsil of a healthy dog, similar to humans.