Transposable elements are DNA fragments able to transpose from one locus of the genome to another, and to modify the expression of genes in which they insert. In plants, two classes of transposable elements have been isolated. "Classical" elements use a conservative mode of transposition: the element is excised from its original insertion site, and reinserted into a new location. In contrast, elements of the retrotransposon type move following a replicative mode of transposition, via a RNA copy of the original element, which remains inserted in its original insertion site. The mutations created by retrotransposons are therefore stable. Transposition is not a random event, and transposable element activity is regulated by the element itself, as well as the host plant and by environmental conditions. Transposable elements also constitute very interesting tools for plant genetic engineering, mostly for the cloning of genes by gene-tagging procedures, which allow the cloning of genes whose function is not known, but for which mutant phenotypes can be screened. Isolation of transposable elements from the tobacco ( Nicotiana tabacum ) genome will give powerful tools for cloning by gene-tagging in this species. In the laboratoire de Biologie Cellulaire at the INRA in Versailles, eight spontaneous NR- mutants, induced by transposable element insertions into the nitrate reductase (NR) gene, were obtained from a tobacco mutant line carrying a somatic instability typical of transposable element activity. In three of these mutants, the NR- phenotype is associated with the insertion of copia-type retrotransposon, named Tnt l, and in two others, with the insertion of an other retrotransposon, Tto l. A third type of element was isolated from a sixth mutant, and its characterization is under way. The functional study of this new element will allow to determine if it is involved in the somatic instability, and might allow the creation of a new gene-tagging tool in tobacco, for the cloning of genes coding for biochemical functions whose product is poorly accessible, and for which efficient selective screens at the cellular level are available.