Genus Bacillus contains ubiquitous, Gram positive bacteria which can be found in the rhizosphere coexisting with other bacteria. Research on the interactions between these bacteria in rhizosphere is very important. Interactions may be cooperative or antagonistic depending on how bacteria recognize and discriminate each other in a process called kin discrimination. An example of bacterial cooperative behaviour is swarming, a multicellular movement of flagellated bacteria over semi-solid surfaces. Bacillus species can also produce biofilm and colonize plant roots. The aim of this master’s thesis was to determine whether B. subtilis tomato rhizosphere isolates recognize each other as ''kin'' or ''non-kin'' on semi solid agar and to test whether ''non-kin'' isolates, compete and exclude each other on the root surface. The root colonization assay on Arabidopsis thaliana was optimized to test the competition between the isolates. Our results showed that most of the isolates recognized each other as ''non-kin'', which was visible as a boundary line between the swarms on semi-solid agar plate. The only ''kin'' strain pair that merged was T16-8 and PS-216. Furthermore, the inoculation delay of one strain and the distance between the inoculation sites did not have an impact on the formation of the boundary line. In contrast, phosphate starvation caused a visible difference in bacterial growth and swarm morphology. Growth was significantly weaker with less visible boundary lines when no phosphate was present. Furthermore, when ''kin'' strains were inoculated on the root in the colonization assay, we observed the presence of both strains, while only one strain prevailed when ''non-kin'' strains we coinoculated on the plant root. We successfully optimized a method for root colonization assay of A. thaliana, which is important for the future studies of interactions between bacteria on the plant roots.