The cDNA and genomic sequences of IgZ-2 and IgM-IgZ were characterized in grass carp Ctenopharyngodon idellus in the present study.The secretory IgZ-2(sIgZ-2)cDNA sequence of grass carp is 1 889 bp,encoding 539 amino acides,and it contains a 267 bp 3′ untranslated region(UTR),but lacking 5′ UTR and partial variable region(V).The secretory IgM-IgZ(sIgM-IgZ)cDNA sequence of grass carp is 1 316 bp,encoding 361 amino acids,and contains a 3 bp 5′ UTR and a 227 bp 3′ UTR.The cDNA sequence of membrane-bound form IgM-IgZ(mIgM-IgZ)possesses two transmembrane exons,which are also spliced to a site within the CH2 exon.Amino acid alignment based on heavy chain constant regions of Igs showed that IgZ-2 and IgM-IgZ contain conserved cysteines.Phylogenetic analysis indicates that the grass carp IgZ-2 was clustered closely with the reported zebrafish IgZ, and then with IgZ in grass carp,which then was clustered well with grass carp IgM-IgZ and chimeric IgM-IgZ in common carp.There was apparent discrepancy in mRNA expression of sIgM-IgZ,mIgM-IgZ and sIgZ-2 in the four fish.In general,IgM-IgZ and IgZ-2 were expressed mainly in lymphoid organs.