This study was carried out in 2017-2018 to determine the in vitro and in vivo activities of some bacterial bioagents against Sclerotinia sclerotiorum, which causes root and root-collar rot in sunflower cultivation areas of Konya and Aksaray provinces. Against the two most virulent S. sclerotiorum (Hırkatol and Eskil) isolates selected as a result of pathogenicity tests after being isolated and diagnosed from diseased plants which were collected from sunflower cultivation areas in Konya and Aksaray provinces, the antifungal effects of 16 bacterial isolates from the soil in the rhizosphere region of the healthy sunflower plants from the same areas were evaluated. Primarily, the most effective bacterial bioagents were determined by dual culture tests. As a result of the in vitro tests, a total of 5 bacterial isolates constituting the largest zone diameter were molecularly identified according to 16S rRNA and were used in pot experiments. The bacteria were identified as Bacillus cereus, Bacillus simplex, Brevibacterium frigoritolerans, Bacillus toyonensis (2 isolates) and were coded using the BLAST program of the GenBank database (NCBI). As per in vitro, the highest effect in both isolates of S. sclerotiorum was observed in Bacillus cereus and Bacillus simplex with an inhibition rate of 49.19-57.95%. Except for Bacillus toyonensis (B1), one of the bacterial species which were tested in vivo, all the bacteria reduced or stopped lesion development compared to the control. As a result of the application, the biological control agent completely prevented the growth of both the isolates of Bacillus cereus and Bacillus simplex S. sclerotiorum in in vivo conditions (100%). Efficacy studies have shown that bacterial isolates both cause healthy growth of sunflower plants and significantly prevent disease formation in treated plants when compared to control plants. These results emphasize the importance of such studies as a tool for the development of sustainable agricultural practices that can be easily applied in our region, and also show that B. cereus and B. simplex in sunflowers can be potential bacterial bioagents that can be used in biological control against S.sclerotiorum. In addition, it will be useful to carry out studies on the development of commercial preparations of the bacterial isolates found in the study.