We examined the resistance phenotype of a large number of transgenic tobacco plants originating from 12 commercial (Nicotiana tabacum) cultivars expressing the sense form of the nucleoprotein (N) gene of L3, a Bulgarian isolate of tomato spotted wilt virus (TSWV). The analysis revealed that transgenic plants are completely protected against the homologous L3 isolate of TSWV irrespective of whether or not they contain detectable levels of translational product. The effectiveness of protection against the virus was investigated upon mechanical inoculation under greenhouse conditions and in field trials. Non-segregating resistant lines were selected and the inheritance of the resistance to TSWV was analysed in successive generations (R3–R<6). Extensive tests under controlled conditions and two-year field trials proved that the resistance to TSWV is stable in different environments and is a stably inherited trait.