Terminal differentiation of many cell types involves permanent withdrawal from the cell division cycle. The p18 INK4c protein, a member of the p16/INK4 cyclin-dependent kinase (CDK) inhibitor family, is induced more than 50-fold during myogenic differentiation of mouse C2C12 myoblasts to become the predominant CDK inhibitor complexed with CDK4 and CDK6 in terminally differentiated myotubes. We have found that the p18 INK4c gene expresses two mRNA transcripts—a 2.4-kb transcript, p18 (L), and a 1.2-kb transcript, p18 (S). In proliferating C2C12 myoblasts, only the larger p18 (L) transcript is expressed from an upstream promoter. As C2C12 cells are induced to differentiate into permanently arrested myotubes, the abundance of the p18 (L) transcript decreases. The smaller p18 (S) transcript expressed from a downstream promoter becomes detectable by 12 h postinduction and is the predominant transcript expressed in terminally differentiated myotubes. Both transcripts contain coding exons 2 and 3, but p18 (L) uniquely contains an additional noncoding 1.2-kb exon, exon 1, corresponding exclusively to the 5′ untranslated region (5′ UTR). The expression pattern of the shorter p18 (S) transcript, but not that of the longer p18 (L) transcript, correlates with terminal differentiation of muscle, lung, liver, thymus, and eye lens cells during mouse embryo development. The presence of the long 5′ UTR in exon 1 attenuated the translation of p18 (L) transcript, while its absence from the shorter p18 (S) transcript resulted in significantly more efficient translation of the p18 protein. Our results demonstrate that during terminal muscle cell differentiation, induction of the p18 protein is regulated by promoter switching coupled with translational control.