The callose deposition on the cell walls of one susceptible and two resistant tobacco varieties, infected with P. tabacina Adam was investigated using the fluorescent technique. It was established that the callose papillae didn't appear in the cotyledons of susceptible genotype grown on Knop's solution and were slightly expressed in the susceptible MgO-grown cotyledons. In both resistant varieties an intensive callose formation was observed as a response to the fungal infection. It was less prominent in the Knop grown resistant cotyledons than in the MgO-grown ones. In the MgO-cultivated cotyledons where the blue mold resistance genes are completely expressed the cells with callose ingrowths were more numerous and their walls were thicker compared to the susceptible variety. It was speculated that the callose depositions observed after pathogen invasion are morphological barriers of plant against the progress of the parasite and that the systems of recognition are i mportant for their formation.