The molecular nature of Spemann's organizer phenomenon has long attracted the attention of embryologists. goosecoid is a homeobox gene with a DNA-binding specificity similar to that of Drosophila bicoid. Xenopus goosecoid is expressed on the dorsal side of the embryo before the dorsal lip is formed. Cells expressing goosecoid are fated to become pharyngeal endoderm, head mesoderm and notochord. Transplantation of goosecoid mRNA to the ventral side of Xenopus embryos by microinjection mimics the properties of Spemann's organizer, leading to the formation of twinned body axes, goosecoid is activated by dorsal inducers and not affected by ventral inducers. In the mouse, goosecoid is expressed in the anterior tip of the primitive streak. The availability of two early markers, goosecoid and Brachyury, opens the way for the comparative analysis of the vertebrate gastrula. The results suggest that the goosecoid homeodomain protein is an integral component of the biochemical pathway leading to Spemann's organizer phenomenon.