Several bacteria are known as the causal agents of diseases of the cultivated button mushroom(Agaricus bisporus)and oyster mushroom(Pleurotus ostreatus). Pseudomonas tolaasii is the causal agent of brown blotch disease of commercialmushrooms. Pseudomonas sp. HC1 is a potent biological control agent to control brown blotch disease caused byPseudomonas tolaasii. This can markedly reduce the level of extracellular toxins (i.e., tolaasins) produced by Pseudomonastolaasii, the most destructive pathogen of cultivated mushrooms. To define the optimum conditions for the mass production ofthe Pseudomonas sp. HC1, we have investigated optimum culture conditions and effects of various nutrient source on thebacterial growth. The optimum initial pH and temperature were determined as pH 5.0 and 20^oC, respectively. The optimalculture medium for the growth of tolaasin inhibitor bacterium was determined as follows: 0.9% dextrin, 1.5% yest extract,0.5% (NH₄)₂HPO₃, 4mM FeCl₃, and 3.0% cysteine.