1. This paper deals with the results of the writer's investigations on the influence of hydrogenion concentration upon the antagonistic action of Streptomyces allus (Strain No.5) against three species of plant disease fungi, Ophiobolus Miyabeanus, Diaporthe Batatatis and Corticium centrifugum on culture media. 2. The optimum hydrogen-ion concentration for the growth of the mycelium of Ophiobolus Miyabeanus seems to lie at pH6.2, and for that of Diaporthe Batatatis at pH5.2-6.2, but they grew slightly at pH3.0 and fairly well at pH8.6. 3. The relation of pH value of culture media to the growth of Streptomyces albus was also studied by growing the fungus on poured plates of Czapek's agar adjusted at different hydrogenion concentrations. It was found that the organism in culture media grew at the range of pH6.0 to ca. 8.6, and the optimum pH value of media for its growth seemed to lie at approximately pH7.6. 4. By using Czapek's agar medium, the effect of mix culturing three fungi with the Streptomyces at various pH values on the width of sterile region appearing around the colony of Streptomyces was studied. The inhibitor's inoculum was placed on the center of each Petri dish, and the fungus to be tested was transplanted on the place of seven centimetres away from it at the same time. The antagonistic action of streptomyces allus against the three fungi was clearly influenced by the pH value of the culture media. No antagonistic action of the inhibitory organism was shown to all the strains of Corticium centrifugum. However, the inhibition of growth of Ophiobolus Miyabeanus takes place between pH3.0 and ca. 8.6, and of Diaporthe Batatatis between pH4.0 and ca. 8.6. The optimum pH for the inhibitory action seems to lie at approximately pH6.6-7.6. 5. The activity of the antibiotic substances produced by Streptomyces allus was influenced by the pH value of culture media. It was intensive at between pH4.0 and ca. pH8.6. The maximum activity of this substance was shown at pH6.6-7.6. No sign of activity of the antibiotic substance, however, was shown at pH3.0. 6. From the result of the experiments, the writer came to the conclusion that in the practice of biologic control of crop-diseases the hydrogenion concentration in the antagonistic complex must be considered.