The rhizosphere has a special position in soil biology, due to specific interactions between soil microorganisms and tobacco plants. During their growth tobacco plants compete with soil microorganisms for nutrition. However relations between plants and microorganisms go beyond this competition for nutrition. The action of microorganisms on vegetal and animal remains and insoluble mineral compounds leads to the release of assimilable substances for the tobacco plants. Through their vital activity, some microorganisms, especially bacteria, release biologically active substances around them that exert a favourable action on the plants or produce antibiotic substances with inhibitory action for pathogenic micromycetes from the roots of tobacco plants. In researches conducted between 1993 and 1996 the rhizosphere and edaphosphere of the tobacco plants were studied in the definite conditions of the attack of some pathogenic micromycetes on the roots in the seedbeds ( Chalara elegans , Rhizoctonia solani , Pythium de baryanum, Alternaria sp.). When providing the best conditions for the development of the tobacco seedlings (nutritive substances, moisture, pH, aeration), the plants, through the substances released by their roots and the exfoliation of the radicular and of the epidermic cells, create in the radicular system an ecological environment fundamentally different from the ordinary one found in the rest of the soil. The microflora in the rhizosphere multiply in a differential way, in relation with its capacity to use for nutrition the substances released by the roots. During the competition among the different microbial species, it was found that bacteria with simple morphology always win, using sugars and aminoacids released by the roots, and those bacteria which are able to synthesize their growing factors needed for their multiplication. The most frequent species of bacteria in the rhizosphere of tobacco plants were Pseudomonas fluorescens , Arthrobacter sp. and Bacillus circulans . The research proved that in all the cases when the populations of the rhizospheric bacteria were numerous, the degree of attack of the pathogenic micromycetes in the seedbeds with tobacco seedlings is highly diminished even in the case of artificial infection.