A gram-negative bacterium was isolated from spent substrates of and showed significant antagonistic activity against . The bacterium was identified as sp. based on cultural, biochemical, physiological characteristics and a 16S rRNA sequence analysis. The isolate is saprophytic, but not parasitic or pathogenic on cultivated mushroom, whereas it showed strong inhibitory effects against cells . The control efficacy of sp. HC12 against brown blotch of was up to 63% on . The suppressive bacterium may be useful for the development of biocontrol systems.