The ecology of the pathogen in soil and in tobacco plants was investigated by using a newly devised selective medium. Control by cultural practices and a new soil sterilant were investigated. The pathogen was able to survive in soil for more than 4 months at 10-20.degree.C, whereas its population decreased gradually at below 5.degree.C. When the infested stems were buried into upper layer of field soil, the pathogen moved only deeper. In the case of stems buried in deep soil, the pathogen moved both downward and upward, but not much laterally. In tobacco plants, the pathogen moved more rapidly in the stem than in the root, and in both tissues the movement of the pathogen was accelerated by higher temperature; however at 25-30.degree.C. it was slower in resistant cultivar (Awa) than in susceptible one (BY-4). When the soil was flooded after application of rice straw or fresh grass into soil in hot summer the population of the pathogen decreased dramatically. The effect was not observed in the case of matured compost. In the field experiments, the application of grass at amount of 1 000 kg/ha followed by flooding in hot summer showed remarkable effect in controlling bacterial wilt. And good control of bacterial wilt was achieved with application of Dazomet granular at rates of 20-30 kg/ha followed immediately by incorporation into infested soil.