Cathepsin S is a lysosomal cysteine endopeptidase of the papain family. Our preliminary results showed the up-regulation of cathepsin S (CTSS) transcript during the early stage of Edwardsiella ictaluri infection, leading us to speculate that CTSS may play a role in infection. In this report, we identified, sequenced and characterized the channel catfish CTSS cDNA. Total RNA from tissues was isolated and cDNA libraries were constructed by the rapid amplification cDNA end (RACE) method. The gene-specific primers in conjunction with the RACE primers were used to PCR amplify 5'- and 3'-ends of the CTSS transcript. The complete channel catfish CTSS cDNA comprised 1530 nucleotides including a 96-nucleotide 5'-untranslated region (UTR), a 990-nucleotide open reading frame and a 444-nucleotide 3'-UTR. The open reading frame appears to encode a protein of 329 amino-acid residues with calculated molecular mass of 36.7kDa and pI of 5.96. The degree of conservation of the channel catfish CTSS amino-acid sequence in comparison to other species ranged from 56.6 to 68.5%. These results provide important information for further exploring the roles of channel catfish CTSS in antigen processing.