ATHB13 belongs to a family of homeodomain leucine zipper (HDZip) transcription factors in Arabidopsis thaliana. To understand the temporal and spatial distribution of ATHB13 gene expression, we examined the ATHB13 promoter activity by means of fusions to the uidA (GUS, beta-glucuronidase) reporter gene in transgenic plants. The strongest promoter activity was detected in the vasculature of the basal portion of petioles for both rosette leaves and cotyledons and at the base of cauline leaves. Activity was also detected in the stem at the base of the cauline leaf in an area corresponding to the leaf gap in the vasculature. In flowers, promoter activity was also present in the receptacle and in the stigma. Transformation of the same promoter-GUS construct into hybrid aspen (Populus tremula x P. tremuloides) resulted in an analogous expression pattern in the petioles of leaves. The similarity of these expression patterns indicates that the trans-acting factors responsible for ATHB13 expression are conserved between aspen and Arabidopsis. The conserved expression pattern of the highly specific Arabidopsis ATHB13 promoter in hybrid aspen demonstrates the potential utility of Arabidopsis promoters for tissue-specific expression in angiosperm trees.