Six species of soil fungi were studied in association with Helminthosporium sativum. Phoma humicola, Epicoccum purpurascens, and Trichoderma viride strongly inhibited the pathogenic activities of H. sativum, while Actinomucor repens, Sclerotinia trifoliorum, and Myrothecium verrucaria were only slightly inhibitory. The pathogenicity of H. sativum was increasingly depressed by each of the six fungi as soil temperature was increased from 15° to 26 °C. The pH shifts which these fungi induced in soil were not sufficient to be considered as a factor in depressing pathogenicity. In plate tests on nonenriched soil-extract medium, S. trifoliorum and A. repens did not appreciably affect H. sativum. The other four fungi all caused distortion and breakdown of the spores, while M. verrucaria and E. purpurascens were found as internal parasites in spores of H. sativum. P. humicola, E. purpurascens, and T. viride caused the breakdown of mycelium of the pathogen, and P. humicola and E. purpurascens were found as internal parasites of the mycelium. The fungi produced substances in the medium which adversely affect the germination and growth of germ tubes of spores of H. sativum. A. repens and S. trifoliorum were least effective, T. viride and P. humicola were intermediate in their action, and E. purpurascens and M. verrucaria were very severely limiting. Two antagonistic mechanisms, antibiosis and direct parasitism, were demonstrated, both of which were responsible for disorganization of the mycelium of the pathogen. Only those fungi that caused disruption of the mycelium were able to depress the pathogenicity of H. sativum appreciably.