The potential of human embryonic stem (hES) cells to differentiate into cell types of a variety of organs has generated much excitement over the possible use of hES cells in therapeutic applications. Of great interest are organs derived from definitive endoderm, such as the pancreas. We have focused on directing hES cells to the definitive endoderm lineage as this step is a prerequisite for efficient differentiation to mature endoderm derivatives. Differentiation of hES cells in the presence of activin A and low serum produced cultures consisting of up to 80% definitive endoderm cells. This population was further enriched to near homogeneity using the cell-surface receptor CXCR4. The process of definitive endoderm formation in differentiating hES cell cultures includes an apparent epithelial-to-mesenchymal transition and a dynamic gene expression profile that are reminiscent of vertebrate gastrulation. These findings may facilitate the use of hES cells for therapeutic purposes and as in vitro models of development.