We have characterized an Arabidopsis homeobox gene coding for a putative DNA binding protein that represents an early marker for vascular development. The full-length cDNA encodes a protein of 833 amino acids that we have designated Athb-8; it contains the conserved DNA binding domain that characterizes the HD-Zip family of transcription factors. RNA analysis showed that the Athb-8 gene is expressed during the vegetative and the reproductive phases of plant growth. A higher steady-state level of the Athb-8 mRNA was found in flowering stem and root. In situ mRNA analysis of Arabidopsis plants demonstrated that Athb-8 expression is restricted to the procambial cells of embryo and developing organs. Moreover, Athb-8-GUS expression was found in single parenchyma cells which are differentiating into tracheary elements in wounded tobacco transgenic plants. Finally, we showed that the auxin, indole-3-acetic acid, which is involved in vascular development and differentiation, modulates the expression of the gene. Taken together, these results suggest that Athb-8 might be a regulator of vascular development in Arabidopsis thaliana.