The technique of homologous cloning and Rapid Amplification of cDNA Ends(RACE) was used to amplify full length cDNA gene of immunoglobulin light chain(κ chain) from cobia ( Linnaeus). The full length cDNA of κ in cobia is 969 bp, containing a 3′ untranslated region (UTR) of 188 bp, a 5′ UTR of 52 bp, and an open reading frame (ORF) of 729 bp, encoding 242 amine acids. The estimated molecular weight of Ig κ is 26.255 kD, and the theoretical isoelectric point is 7.52. The deduced Igκ amino acid sequences of cobia were compared with those of other teleost species. For the constant region of Igκ, higher percentage similarity was obtained from comparisons between and between andhigher percentage similarity was obtained from comparisons between R. canadium By the phylogenetic tree of immunoglobulin light chain constant region, Igκ amino acid sequences of cobia were clustered with IgL thwere clustered together. The expression of Igκ gene in healthy cobia was initially measured by existed more obviously in liver and gill than in other tissues, but they were hardly expressed in intestine and gill increased obviously after cobia was immunized by intraperitoneal injection withThe and gill are main organs for Igκ production after and play a critical role in host-pathogen interaction