The variability of P. tabacina A. populations was studied by using the isoenzymes of peroxidase (PO), malate dehydrogenase (MDH) and superoxide dismutase (SOD) as molecular markers. P. tabacina conidia/conidiophores from N. tabacum crops cultivated in distant areas of Europe (France and Bulgaria) were investigated during a long period of time (1978-1992). It was found that no variations of P. tabacina as a function of space and time occurred. The data point to the genetical stability of the presently spread strain of the pathogen and the lack of processes of new strain formation. However, a significant variability of P. tabacina as a function of host plant was established. Suspension of conidia/conidiophores of P. tabacina originating from N. tabacum (P 48 cv.) was used to inoculate the wild species N. repanda. First conidia/conidiophores of P. tabacina produced on the new host plant (designated as Pt/Nr) as well as those from the “classical” host, N. tabacum (designated as Pt/Nt) were analysed. Thus, two types of P. tabacina (according to their origin) were compared. It was shown that one MDH and most PO isoenzymes present in Pt/Nt were not observed in Pt/Nr; these isoenzymes repressed following the transfer were common for Pt/Nt and its N. tabacum host. Moreover, in conidia/conidiophores of Pt/Nr new isoenzymes appeared, namely one major PO, one MDH and a block of three SOD isoenzyme components. These isoenzymes were common for Pt/Nr and the new host, N. repanda. It is noteworthy that all fungus specific isoenzymes present in Pt/Nt (one PO, five MDH and seven SOD components) were conserved in Pt/Nr. The results reveal molecular mechanisms underlying relationships between host plants and obligatory fungal pathogens, such as P. tabacina. They could be used in blue mold epidemiology research.