Mushroom is cultivated in many areas of the Korea as one of the major economical crops. The production areas have steadily increased approximately 3,674 ha in 2002 to 4,118 ha in 2005. Several bacteria have been known as the causal agents of certain diseases of cultivated button mushroom, Agaricus bisporus, and oyster mushroom, Pleurotus ostreatus. It is well known as bacterial diseases of the cultivated mushroom such asbrown blotch, mummy disease, bacterial pit, bacterial rot and weeping disease, ginger blotch, and drippy gill. Unknown soft rot bacterium was isolated from sunken browning symptom of cultivated oyster mushrooms grown in Korea. The symptoms are appear as a sunken browning lesions on the caps of affected mushrooms. The bacterium causes a rapid soft rot of cultivated mushrooms in comparison with brown blotch bacteria at temperatures above 25℃. From these lesions we isolated one bacterial strain (designated OM1). Inoculation of bacterial isolates into mushroom caps yielded characteristic sunken brown, watersoaked and severe soft rot symptoms, but which were indistinguishable in early stage from those of the bacterial brown blotch well known to mushroom growers. Results of Gram stain and biochemical tests identified this isolate as Burkholderia gladioli pv. agaricicola. This was confirmed by pathogenicity, physiological and biochemical characteristics, and results of an analysis of the 16S rRNA gene sequences and the fatty acids profile. This is the first report of the isolation of B. gladioli pv. agaricicola from cultivated oyster mushroom in Korea.