Mushroom flies are one of the most serious threats to mushroom production and cultivation in Korea, and they cause significant losses in yield and quality. In this study, we investigated the occurrence and control of mushroom flies in farms where Agaricus bisporus was cultivated in Chungnam. The cultivation period was more than 7 years in the 32 mushroom farms examined; about 28% were environment-friendly cultivation farms, and 72% were conventional cultivation farms. Mushroom flies were a major pest in about 84% of the examined farms. Most of the mushroom flies were sciarids and phorids (71% and 24%, respectively). The adult flies were found throughout the year, but their occurrence was the highest in September when the temperature increased. In general, the occurrence of mushroom flies in the examined farms was high; however, insect nets and post-crop steaming programs used as basic control methods were not implemented properly. Our findings show that integrated pest management of mushroom flies is urgently required for A. bisporus cultivation in Korea.