The gene rolB from the T-DNA of Agrobacterium rhizogenes is capable of directing differentiation of roots in transformed plant cells. A detailed histological analysis of the expression of this gene in transgenic tobacco plants is presented here. Gene fusions between the promoter of rolB and the GUS reporter gene (encoding E. coliβ-glucuronidase) were utilized: tissue and cell specific GUS activity as driven by a long (1185 base pairs) and a short (306 bp) version of the rolB promoter was visualized histochemically during different phases of plant development, from embryo to flowering. The extended (1185bp) rolB promoter is under developmental control and is specifically active in the initial cells of all types of meristems and in the vascular system (phloem, phloem parenchyma, xylem parenchyma, pericycle and pericycle-like cells) of mature organs as well as in the central cylinder of the embryo. In contrast, the 306 bp deletion of the promoter is unable to drive expression in menstematic initials but is still active in the vascular system of the seedling.