We present a list of the beetle species associated with the oyster mushroom, Pleurotus ostreatus Fr. in the birch forest of Onon-Balj National Park in Mongolia. A total of 21 species in seven families were found in this fungus, including fungivorous species: Litargus japonicus Reitter and Mycetophagus antennatus (Reitter) (Mycetophagidae), Pseudamblyopus similis (Lewis) (Erotylidae), and Lordithon sp. (Staphylinidae) etc. Both adult and larval forms are presented in the checklist along with the type of fungal association, of which approximately 52 percent are obligate mycetobionts. The main trends of the ecological and trophic specialization of the mycetophilic beetles are discussed.