Cofilin/ADF proteins are a ubiquitously expressed family of F-actin depolymerizing factors found in eukaryotic cells including plants. In vitro, cofilin/ADF activity has been shown to be essential for actin driven motility, by accelerating actin filament turnover. Three actin depolymerizing factors (n-cofilin, m-cofilin, ADF) can be found in mouse and human. Here we show that in mouse the non-muscle-specific gene-n-cofilin-is essential for migration of neural crest cells as well as other cell types in the paraxial mesoderm. The main defects observed in n-cofilin mutant embryos are an impaired delamination and migration of neural crest cells, affecting the development of neural crest derived tissues. Neural crest cells lacking n-cofilin do not polarize, and F-actin bundles or fibers are not detectable. In addition, n-cofilin is required for neuronal precursor cell proliferation and scattering. These defects result in a complete lack of neural tube closure in n-cofilin mutant embryos. Although ADF is overexpressed in mutant embryos, this cannot compensate the lack of n-cofilin, suggesting that they might have a different function in embryonic development. Our data suggest that in mammalian development, regulation of the actin cytoskeleton by the F-actin depolymerizing factor n-cofilin is critical for epithelial-mesenchymal type of cell shape changes as well as cell proliferation.