Myogenin, a member of the MyoD family which governs skeletal muscle differentiation, was identified as a pair of phosphorylated bands on SDS-PAGE during myogenesis. The slow migrating form was found to be hyperphosphorylated myogenin. In vitro phosphorylation by CDC2 kinase caused a prominent reduction in electrophoretic mobility of myogenin. Furthermore, we demonstrated that phosphorylation of the serine residue at position 43 contributes to the modification of myogenin in vivo and in vitro resulting in the reduction in electrophoretic mobility. We propose here that a CDC2-like proline-directed kinase regulates myogenin activity through its phosphorylation.