As shown in the accompanying paper, c chains of the membrane-bound(pm) and secreted (p5) forms of IgM are encoded by two species of mRNA. Cloned cDNAs produced from the two p mRNAs of M104E mouse myeloma tumors differ only at their 3’ends, which encode either the p,,, or ps C terminus. In this paper, we show that both p,,, and ps mRNAs are produced from transcripts of a single p gene. The last 187 nucleotides of ps mRNA are derived from DNA contiguous with the 3’end of the sequence encoding the C, 4 domain. The p,,, cDNA clone does not include these 187 nucleotides, but instead contains 392 nucleotides derived from two exons located 1850 bp 3’to the C, 4 sequence. Comparison of genomic and cDNA sequences show that in CL,,, mRNA, an RNA splice of 1850 nucleotides joins a site in the coding sequence at the end of C, 4 with a site at the beginning of the first membrane-specific exon. A second RNA splice of 118 nucleotides joins sequences transcribed from the first and second membrane-specific exons. The differences observed between p,,, and~ 1~ cDNAs suggest that developmental control of the site at which poly (A) is added to transcripts of the p gene determines the relative levels of pdm or ps chain synthesis. We discuss possible models for the control of p gene transcripts and the significance of this form of developmentally regulated RNA processing for the evolution of eucaryotic “split genes.”