To elucidate the structure of the gene for the constant region of immunoglobulin mu chains, we have cloned a 9.9-kilobase-pair fragment of mouse DNA bearing a gene for the constant region of the mu chain (C mu gene) from an IgM-secreting mouse plasmacytoma. The sequence around this gene has apparently undergone somatic rearrangement; the gene occurs in an EcoRI restriction endonuclease fragment of a different size from that in embryo or liver DNA and no C mu-bearing fragment of embryo size remains in the plasmacytoma. The cloned sequence lacks a variable region gene; hence, if this C mu gene is active, its position within the clone indicates that the gene for the variable region of a heavy chain (VH gene) must be more than 3.7 kilobase pairs away. The C mu gene is divided by three intervening sequences into four coding segments, each of which encodes one of the domains (homology units) of the polypeptide. The nucleotide sequence coding for amino acids near the V-C junction is not present within the C mu clone or clones bearing homologous embryonic VH genes. This suggests that an immunoglobulin heavy chain, in common with light chains, is encoded not only by a V and C gene, but also by an independent joining region (JH) gene.