Abstract: Two biologically active strains of Bacillus circulans (SF1 and DE62), described in Egypt as silicate dissolving bacteria, were experimented for the control of potato brown rot caused by Ralstonia solanacearum. In in vitro studies, the isolate SF1 as well as DE 62 were antagonistic to certain isolates of the pathogen. Both spore forming isolates produce nitrogenase, hydrolytic enzymes and mobilize potassium. Our results show in general, a tendency of R. solanacearum population to increase in the rhizosphere of potato raised in Bacillus circulans treated soil. Similar trend was observed for the Area Under Disease Progress Curve (AUDPC) and wilt severity. At cultivars level, cv., Cara (late mature) showed highly significant increase in AUDPC and wilt severity over Inova (early mature) and Picasso (medium mature) either in presence or absence of Bacillus circulans. The greater increase of the pathogen, in B. circulans treatments, was attributed to the nitrogen fixation potential and further enrichment of the micro ecosystem, i.e., the rhizosphere, with nutrients due to hydrolytic enzymes produced by the above mentioned strains. Raising potatoes in clay soil with increasing percentage of sand up to 20% revealed a tendency for disease severity decrease. The effect was attributed to an easier flushing of the pathogen under alkaline soil conditions. The significant increase in plant height and weight in the B. circulans treatments was attributed to greater availability of nutrients especially nitrogen and potassium. It could be concluded that the results of antagonism in vitro may be inconclusive under certain circumstances following soil application, in biological control trails. Other inherent physiological and biological activities of the antagonist may be considered as limiting factors in biological control success.