By successive filtration, .a bact e r iophage of high potency was isolated from a culture of Bacterium solanacearum in potato dextrose solution. The phage, judged from the number of plaques formed on agar plates, increased when added to the fresh bacterial culture, and after 15 filtrations inhibited the growth of the bacteria at dilution of 10^-10. In trials using six cultures of Bact. solanacearum, the phage affected only the culture of the organism from which it was isolated. Experimentally, tomato plants were protected from the bacterial wilt by adding the phage to the soil. In the present studies the author recognized t wo distinct strains of Bact. solanacearum: one of them formed round and homogeneous colonies on potato agar and did not liquefy gelatine ; whereas the other strain formed irregular, fluidal and non-homogeneous colonies and liquefied gelatine . The author demonstrated the presence of bacteriophages in the cultures of the latter. The occurrence of the homogeneous strain was observed in the old or phage-added cultures of the fluidal strain and also in the diseased lesion of tomato plants inoculated with the fluidal strain. From these experimental results the author pointed out that such fluidal strain as mentioned above may be a variation caused by the action of bacteriophage.