The aim of the study was to evaluate the effect of Bacillus megaterium ONU 500 on the germination and growth of sunflower seedlings. Materials and methods. Sunflower seeds were sown in containers with soil inoculated with a suspension of bacteria B. megaterium ONU 500 in concentration of 106 CFU/ml. After 21 days growth characteristics of seedlings were compared: germination, average height, root length, leaf area, dry weight. Effect of these microorganisms on biofilm formation on roots was investigated with light microscopy. Results. Inoculation of soil with B. megaterium ONU 500 resulted in the positive effect on roots length (increase by 21%), leaf area (increase by 22%) and dry weight of seedlings (increase by 17.8%). No significant effect on germination and plant height was observed. On the roots of plants grown in soil treated with B. megaterium ONU 500 more developed biofilms with well formed matrix were observed, which indicates the stimulation of ability of plant and soil microbiota to form biofilms in presence of bacilli from the studied strain. Conclusion. The investigations demonstrated that bacteria of B. megaterium ONU 500 strain caused a positive effect on development of sunflower seedlings and formation of biofilms on their roots.