Insulin-like growth factor-I (IGF-I) activates not only the phosphatidylinositol 3-kinase (PI3K)–AKT cascade that is essential for myogenic differentiation but also the extracellular signal–regulated kinase (ERK) 1/2 cascade that inhibits myogenesis. We hypothesized that there must be a signal that inhibits ERK1/2 upon cell–cell contact required for skeletal myogenesis. Cell–cell contact–induced engagement of ephrin ligands and Eph receptors leads to downregulation of the Ras-ERK1/2 pathway through p120 Ras GTPase-activating protein (p120RasGAP). We therefore investigated the significance of the ephrin/Eph signal in IGF-I–induced myogenesis. EphrinA1-Fc suppressed IGF-I–induced activation of Ras and ERK1/2, but not that of AKT, in C2C12 myoblasts, whereas ephrinB1-Fc affected neither ERK1/2 nor AKT activated by IGF-I. IGF-I–dependent myogenic differentiation of C2C12 myoblasts was potentiated by ephrinA1-Fc. In p120RasGAP-depleted cells, ephrinA1-Fc failed to suppress the Ras-ERK1/2 cascade by IGF-I and to promote IGF-I–mediated myogenesis. EphrinA1-Fc did not promote IGF-I–dependent myogenesis when the ERK1/2 was constitutively activated. Furthermore, a dominant-negative EphA receptor blunted IGF-I–induced myogenesis in C2C12 and L6 myoblasts. However, the inhibition of IGF-I–mediated myogenesis by down-regulation of ephrinA/EphA signal was canceled by inactivation of the ERK1/2 pathway. Collectively, these findings demonstrate that the ephrinA/EphA signal facilitates IGF-I–induced myogenesis by suppressing the Ras-ERK1/2 cascade through p120RasGAP in myoblast cell lines.