Using the basic helix-loop-helix domain of the myogenic factor myogenin as a probe, we identified a clone from a sea urchin cDNA library with considerable sequence similarity to the vertebrate myogenic factors. This cDNA, sea urchin myogenic factor 1 (SUM-1), transactivated a muscle creatine kinase-chloramphenicol acetyltransferase reporter gene in 10T1/2 fibroblasts to a level comparable to that of the vertebrate myogenic factors. In addition, bacterially expressed beta-galactosidase-SUM-1 fusion protein interacted directly with the kappa E-2 site in the muscle creatine kinase enhancer core as assayed by electrophoretic mobility shift assays. Stably transfected SUM-1 activated the muscle differentiation program and converted 10T1/2 cells from fibroblasts to myotubes. In sea urchin embryos, SUM-1 RNA was not detected before gastrulation. It accumulated to its highest levels during the prism stage when myoblasts were first detected by myosin immunostaining and then diminished as myocytes differentiated. SUM-1 protein was localized in secondary mesenchyme cells when they could first be identified as muscle cells by myosin immunostaining. These results implicate SUM-1 as a regulatory factor involved in the early decision of a pluripotent secondary mesenchyme cell to convert to a myogenic fate. SUM-1 is an example of an invertebrate myogenic factor that is capable of functioning in mammalian cells.