Pseudomonas tolaasii causes brown blotch disease in cultivated mushrooms by producing tolaasin, a peptide toxin, which forms pores on the membrane and disrupts the cellular and fruiting body structures of mushrooms. For the biological control of this mushroom disease, virulent bacteriophages of P. tolaasii were isolated from the sewage of Cheongju, Korea. Twenty-one phages were isolated from four different locations, and their toxicities to host bacteria were measured by inspecting the turbidity and size of their plaques. They were divided into three categories on the basis of their toxicities to host bacteria. In order to test if these phages can be used for the biological control of mushroom diseases, a pitting test was performed. The surfaces of mushroom caps were inoculated with both pathogenic bacteria and their phages. Phage toxicity was analyzed by measuring the size of the blotches that formed on the surface of mushrooms, because these sizes are representative of the amount of tolaasin peptide produced by pathogenic bacteria in the presence of bacteriophages. The formation of blotches was completely blocked by co-incubated phages. These results show that phages can sterilize pathogenic bacteria in mushroom tissues as well as be useful for the biological control of brown blotch disease. The optimum conditions for the bactericidal activity of the phages were also determined.