After citing the origins and properties of tobaccos created between 1962 and 1968 (P.B. D6 and N.F.B.E.), the authors give two examples of simultaneous selection carried out to associate new hereditary factors. P.N. is a tobacco with low alkaloid content and has gradually been deprived of its ability to produce nornicotine. P.49 has very similar characteristics to those of P.B. D6, but also possesses the resistance gene to Chalara elegans. It was shown that in dark tobaccos, resistance to blue mold, TMV and PVY, as well as to black root rot are not antagonistic. On the other hand, low alkaloid content was correlated to bad colouring of raw tobacco, and the resistance gene to C. elegans (originating from N. debneyi) seemed to be correlated to a sometimes excessive number of days to flowering.