A gram-positive bacterium was isolated from the spent substrate of Agaricus bisporus that showed a marked antagonistic activity against Pseudomonas agarici. It was identified as Bacillus safensis HC42 based on its cultural, biochemical, and physiological characteristics, and 16S rRNA sequence. B. safensis HC42 was saprophytic, but not parasitic or pathogenic, in cultivated mushrooms and showed strong inhibition of P. agarici in vitro. Moreover, it showed a control efficacy of 66 % against browning disease caused by P. agarici in Agaricus bisporus. Therefore, B. safensis HC42 may be useful in the future for the development of a biocontrol system.