Tobacco mildew, which is caused by an Oomycetes fungus known as Peronospora tabacina A. (or P. hyoscyami) is a permanent threat in many tobacco-growing countries worldwide and can cause very severe losses in seed plantations and in the field. Only the species of Australian origin and belonging to the Suaveolentes section appear to provide sources of resistance which can be transferred to the industrial varieties. Of the species studied, N. exiqua, N. debneyi, N. megalosiphon, N. goodspeedii, N. excelsior, N. velutina and N. suaveolens are those which offer the most resistance to P. tabacina. Resistance derived from N. debneyi appears to be polygenic and probably trigenic. This resistance is never total. It develops only after several weeks of vegetation and reaches its peak at the flowering stage. For this reason, the fight against mildew demands the treatment of resistant varieties by fungicides (dithiocarbamates, phenylamides) at the seed stage and after transplantation. The resistance derived from N. debneyi is stable. This source of resistance has been used for more than 20 years in Europe and has not become ineffective as a result of the appearance of any new strain of parasite. Some species (eg. N. megalosiphon) may show total resistance (immunity?) at all stages of development. However, it has not yet been possible to transfer this to the industrial varieties.