, which is distributed along the coast of China, is one of the most economically important marine aquaculture species. With the expansion of breeding, diseases caused by bacteria and viral pathogens have become more widespread and serious. The disease caused by the bacterial pathogen has become an obstacle in the development of half-smooth tongue sole aquaculture. Therefore, it is important to understand the defense mechanisms of the host, , against this pathogen. The CD40 protein, which is a member of the tumor necrosis factor receptor superfamily 5, is an important receptor molecule that plays a key role in the host's immune system and participates in several immune signaling pathways. In this study, using the whole-genome sequence of half-smooth tongue sole, we obtained one homolog using the rapid amplification of cDNA ends technique. The full-length cDNA was 2098 bp long and included an open reading frame (ORF) of 1011 bp, a 5'-untranslated region (UTR) of 44 bp, and a 3'-UTR of 1043 bp. The ORF of encoded a 336-amino acid protein with a predicted molecular weight of 37.27 kDa and a theoretical isoelectric point of 5.355. The 3'-UTR included two instability motifs (ATTTA) and one polyadenylation signal (ATTAAA) located 28 bp upstream of the polyA tail signal (AATAAA). The putative amino acid sequence contained one signal peptide, one trans-membrane region, two N-glycosylation sites, and four conserved cysteine-rich domains. The deduced amino acid sequence of shared 28%-47% identity with the CD40s from other teleost fish, mammals, and amphibians, with higher similarity to CD40s of bony fish. The phylogenetic tree constructed based on the amino acid sequences demonstrated that the CD40 clustered in one branch with other teleost fish. Real-time quantitative PCR analyses detected transcripts in a wide range of tissues in healthy adult fish. There were higher transcript levels of V. harveyi. After challenge with the pathogen, the peak transcript levels of CD40 were at 12 h in the liver, and at 6 h in the spleen and the kidney, before a second peak in transcript levels at 48 h in the three tissues. These results indicate that to the bacterial pathogen .