Formation of mesoderm is a crucial event in vertebrate development, establishing many of the important features of the body. Recent studies have implicated molecules that are similar to growth factors in mesoderm formation in Xenopus, but other gene products involved in this process have yet to be identified. Genetic evidence indicates that in the mouse the T gene (Brachyury) has a role in the formation and organization of mesoderm. Mice homozygous for mutant alleles of the T gene do not generate enough mesoderm, and show severe disruption in morphogenesis of mesoderm-derived structures, in particular the notochord. The cloning of the T gene has now allowed us to examine its expression pattern. We report that T-gene expression occurs in both early stage mesoderm and its epithelial progenitor, and then becomes restricted to the notochord. This expression pattern correlates with the tissues affected in the T-gene mutant, and indicates that the T gene has a direct role in the early events of mesoderm formation and in the morphogenesis of the notochord.