Homeodomain-leucine zipper (HD-Zip) proteins are transcription factors as yet found only in plants. We have characterized one HD-Zip gene, ATHB6, from Arabidopsis thaliana. ATHB6 was expressed constitutively in seedlings, but significantly up-regulated in seedlings subjected to water deficit, osmotic stress or exogenous treatment with abscisic acid (ABA), an induction being detectable within 30 min. The ATHB6 induction was impaired in the two ABA-insensitive mutants, abi1 and abi2, but unaffected in the abi3 mutation. The induction was ABA-dependent, since no increase in ATHB6 transcript was detected in the ABA-deficient mutant aba-3 subjected to drought treatment. These results suggest that ATHB6 may act downstream to both ABI1 and ABI2 in a signal transduction pathway mediating a drought stress response. A translational fusion of the ATHB6 promoter with the reporter gene GUS (ATHB6::GUS) in transgenic A. thaliana plants showed high-level expression in leaf primordia. Expression in developing cotyledons, leaves, roots and carpels was restricted to regions of cell division and/or differentiation. The expression in the cotyledons was detectable in the epidermis and high in the stomatal cells. In mature cotyledons and leaves the marker gene was expressed only in the vascular tissue. These expression data suggest ATHB6 to have a function related to cell division and/or differentiation in developing organs.