Abstract:Endophytic Bacillus subtilis strain B47,which was isolated from the stem of tomato, was an efficient biological control agent on tomato bacterial wilt. A strain resistant to streptomycin derivative of wild strain B47 was used to study the colonization ability and fluctuation of populations in soil and tomato organs, and its biological control on tomato bacterial wilt. The result indicated that strain B47 could colonize in both soil and tomato organ. After being inoculated in soil, the population of strain B47 increased gradually from the 15th day to 45th day, and then decreased after 45 days. The colonization ability of strain B47 in soil varied with the treatments and types of soil. The population of strain B47 in sterile garden soil was 9.84×107cfu/g, however, the population in raw garden soil was 9.91×105cfu/g on 45th day after being inoculated. After dipping root inoculation treatment, the population of strain B47 inside roots and stems increased gradually from the seedling stage to fruit setting stage, and then decreased after ripening stage. With the increasing of the population of strain B47, the population of Ralstonia solanacearum decreased significantly while they were mixed at a similar population in the inoculation into soil. The control efficacy on tomato bacterial wilt was 79.79% on 20th day after being inoculated with Ra- lstonia solanacearum by root dip when the population of strain B47 inside roots and stems of tomato plant was 1.17×104cfu/g FW (fresh weight) and 3.33×104cfu/g FW, respectively.