After a brief history of the origins of necrotic PVY strains of tobacco and varieties which are resistant to them, this article looks at the existing relationships between all these forms of resistance which are at times of a recessive nature and their respective efficiency vis-a-vis several virulent PVY isolates. An allelism test has confirmed the presence of this same recessive gene "va" in all the potential genitors of resistance to experimented PVY: VAM, VD, P 48, Havana 307, Havana IIC, Polalta. The level of resistance as regards PVY given by this gene fluctuates from one genotype to another. Consequently, the inoculation of many virulent isolates, which have different levels of agressivity, has allowed us to show 3 allelic forms: va⁰, va¹, va². The form va⁰, is the most efficient, it is present in the VAM genotype while the cultivar VD has the allelic form of va² which is the weakest. This last form should certainly allow the selection of, or induce an easier development of, virulent necrotic lines of PVY.