Myotonic dystrophy (DM1) is a multi-systemic disease caused by a triplet nucleotide repeat expansion in the 3′ untranslated region of the gene coding for myotonic dystrophy protein kinase (DMPK). The primary pathophysiology of DM1 is thought to result from RNA transport and processing defects. The function of DMPK in development or any potential role in DM1 remains unknown. Here we report a novel role for DMPK in myogenesis. We have discovered a specific expression pattern of DMPK in mouse and chick embryonic development. DMPK is expressed in postmitotic cardiac and skeletal myocytes and developmental signaling centers. During cardiac myocyte maturation, DMPK migrates from perinuclear to cellular membrane localization. Manipulating DMPK levels in cultured cardiac and skeletal myocytes has revealed a key role for DMPK in myocyte differentiation. Overexpression of DMPK induces cell rounding and apoptosis in myocytes. In addition, DMPK is necessary for myogenin expression in differentiating C2C12 myoblasts. Developmental Dynamics 237:2353–2366, 2008. © 2008 Wiley-Liss, Inc.