In this study, a CD9 homologue in a reptile, Chinese soft-shelled turtle, has been cloned and identified for the first time.The full-length eDNA of turtle CD9 was 1146 bp and contained a 672 bp open reading frame (ORF) coding for a protein of 224 amino acids.Four transmembrane domains (TMs) divided the turtle CD9 into several parts: short N and C-termini, an intracellular loop and two (small and large) extracellular loops (SEL and LEL).A CCG motif, a potential N-linked glycosylation site and ten cysteine residues were well conserved.The deduced amino acid sequence analysis showed that the turtle CD9 shared 82% identity to duck CD9.Most of the differences were found in the LEL.Phylogenetic analysis showed that the turtle CD9 sequence clustered together with bird CD9 sequence.RT-PCR analysis showed that turtle CD9 was ubiquitously expressed in liver, spleen, kidney, heart, blood and intestine tissues of un-infected turtles.Real-time PCR analysis further indicated that after Aeromonas hydrophila infection, the turtle CD9 mRNA was up-regulated in various tissues at 8 h, and significantly up-regulated during 8 h-7 d.These results indicated that turtle CD9 may be involved in anti-bacterial immune response.