Embryonic mouse neural tubes produce a variety of terminally differentiated cells in vitro, mostly neurons and glia. We report here that some of these cells differentiate into skeletal muscle cells. The possibility of mesoderm contamination was ruled out as follows. First, Dil+ muscle cells were present in cultures from a Dil-labeled neuroepithelium. Second, a small fraction of cultured neural tube cells coexpressed muscle myosin and neuronal pill tubulin within the same cell. Third, embryos generated from embryonic stem cells in which n/acZ was targeted into the myogenic gene myf-5 expressed n/acZ in a localized region of the neural tube. These mvf-5+ cells coexpress neuronal and muscle markers in culture. The developmental significance of this phenomenbn is discussed in the context of overlapping regulatory networks between myogenesis and neurogenesis.