In fish, two types of immunoglobulin heavy chain (IGH) genes, namely, IgM and IgD, have been cloned and characterized. Recently, a new IGH isotype specific to teleosts had been identified from zebra fish, rainbow trout, and fugu. In zebra fish, the domains of this new gene are present upstream of the mu region along the IGH locus. During this study, a novel IGH chimera (IgM-IgZ) has been discovered from common carp. The cloned cDNA encodes a typical leader peptide, a variable region, two constant regions, and a secretory tail. The first constant region is made up of the C(H1) domain of carp IgM, while the second constant region shares a high similarity to the C(H4) domain of the IgZ from zebrafish. Southern hybridization studies of the mu and zeta domains, conducted separately, revealed the presence of at least three copies of the respective genes, and mu and zeta domains might be present on the same loci, although far apart. Expression studies of the IGH genes suggest that there is an increase in chimeric immunoglobulin gene transcription when stimulated with lipopolysaccharide.