Recent data suggest that dental cells utilize the evolutonarily conserved Notch-mediated intercellular signaling pathway to regulate their fates. Here we report on the expression and regulation of Delta1, a transmembrane ligand of the Notch receptors, during mouse odontogenesis.Delta1is weakly expressed in dental epithelium during tooth initiation and morphogenesis, but during cytodifferentiation, expression is upregulated in the epithelium-derived ameloblasts and the mesenchyme-derived odontoblasts. The expression pattern ofDelta1in ameloblasts and odontoblasts is complementary toNotch1, Notch2,andNotch3expression in adjacent epithelial and mesenchymal cells. Notch1 and Notch2 are upregulated in explants of dental mesenchyme adjacent to implanted cells expressingDelta1,suggesting that feedback regulation by Delta–Notch signaling ensures the spatial segregation of Notch receptors and ligands. TGFβ1 and BMPs induceDelta1expression in dental mesenchyme explants at the stage at whichDelta1is upregulatedin vivo,but not at earlier stages. In contrast to the Notch family receptors and their ligand Jagged1, expression ofDelta1in the tooth germ is not affected by epithelial–mesenchymal interactions, showing that the Notch receptors and their two ligands Jagged1 and Delta1 are subject to different regulations.