Infection withAgrobacterium rhizogenes of wounded stems of potato cultivars grown in vitro caused localized prolific root formation (hairy-roots). The cells of these roots contained newly introduced DNA, not detected in normal potato, that was derived fromA. rhizogenes. Single transformed roots from the cultivars Majestic, Record and Maris Bard were regenerated into whole plants. Expression of the introduced genes caused stable alterations in plant development and tuber shape, which were retained under field conditions. The following conclusions were reached. (i) Probably any commercial potato cultivar, cultured under appropriate conditions, is amenable toA. rhizogenes-mediated genetic manipulation. (ii) TheA. rhizogenes-derived genes are convenient model genes for studying questions on structure and expression of transferred genes. (iii) A. rhizogenes-derived genes are of potential use to study the influence of specific genetic factors on complex biological processes such as potato development and tuberization.