The aim of this research is to create isogenic lines of the variety Harmanliska basma 163, with a Nt gene controlling the resistance of tobacco to Nicotiana virus I Smith. At the same time a study of the comportment of the Nt gene, as well as those which control the characteristics of the productivity and the quality in tobacco was carried out. Following repeated back crossing of hybrids, issues of the crossing of Pobeda 3 (a variety resistant to Nicotiana virus I Smith, KOVATCHEVSKI 1983) x Harmanliska basma 163 (standard variety), hybrid isogenes BC1, BC2, BC3, BC4, BC5, and BC6 were produced. In this manner the authors obtained by selection in BC6, tobacco lines with a constant resistance to Nicotiana virus I Smith; resistance controlled by the Nt gene. The analyses of results of productivity and quality of the lines obtained show that they are higher than the standard Harmanliska basma 163 as regards productivity and quality of dry tobacco. The Nt gene did not have an unfavorable effect on these characteristics. The isogenic lines of Harmanliska basma 163 are a bank for genetic studies and for the improvement of varieties.