These experiments were conducted to learn the antibiosis of microfloral organisms in the casing soil to Mycogone perniciosa, and interactions between M. perniciosa and Agaricus bisporus. The results obtained were as follows: 1. In vitro tests, the development of M. perniciosa was suppressed from the unidentified microfloral organisms in the casing soil. All the infected sporophores of A. bisporus occurred within an area applied with spore suspension of M. perniciosa as spot treatment and no infected ones in the area around the spot treated under cropping conditions. 2. In vitro tests, although the antagonistic relationship between M. perniciosa and A. bisporus was somewhat different in varying the kind of media, A. bisporus ultimately overgrew the colony of M. perniciosa. When spore inoculation of M. perniciosa was applied on the surface of grain spawn of A. bisporus and the mid layer of casing soil under cropping conditions, no infected sporophores were produced, whereas the infected sporophores were only produced on casing soil inoculated on the surface of casing soil with spore suspension.