The full cDNA sequence of immunoglobulin M heavy chain of grass carp (Ctenopharyngodon idellus) has been cloned using RACEPCR. It has 1 940 nucleotides, including 5′ UTR of 20 nucleotides and 3′ UTR of 189 nucleotides. Its open reading frame has 1 731 nucleotides which encode a 576 amino acid peptide. The deduced amino acid sequence of grass carp IgM had the highest similarity with Cyprinus carpio (68%). The similarity with Danio rerio, Ictalurus punctatus, Siniperca chuatsi and Gadus morrhua was 61%，43%，33% and 30%, respectively. Clustal X analysis showed that the conserved cysteines and tryptophans exist in grass carp IgM. Phycogenetic analysis placed grass carp IgM and the putative zebrafish IgM into one seperate cluster. The results fro m realtime PCR showed that the grass carp IgM transcripts were detected mainly in lymphoid tissues such as head kidney, kidney and spleen, indicating that the y are main organs for the production of IgM.