Of the five classes of intermediate filaments found in vertebrate tissues, the cytokeratins are considered unique to epithelial tissues, while vimentin is expressed by endothelial and mesenchymal cells. In neither case is the precise function of the filament system known. Epithelial cells in culture often express vimentin as well as cytokeratins, but co-expression in vivo, as reported for pleomorphic adenomas of the parotid gland and metastatic carcinoma cells in ascites or pleural fluid, is still controversial. Here we report the co-expression of cytokeratins and vimentin in situ, in the parietal endoderm of the mouse embryo 8.5-13.5 days old. This population of individual, motile cells seems to be derived from a conventional epithelium by migration and differentiation. Our results support the idea that vimentin expression is specifically related to reduced cell-to-cell contact, and to the independent existence of a cell following detachment from an epithelial sheet.