Twelve species of fungi were isolated from rice straw media for oyster mushroom cultivation. Trichoderma, Aspergillus and Rhizopus were the predominant fungi. Seven species of Trichoderma were isolated and identified from the rice straw media and the order of their frequency in the media was pseudokonigii, aureoviride, viride, harzianum and koningii. Occurrence of harmful fungi in mushroom houses become more severe as the number of cultivation times increased, and that was more severe in spring culture than in autumn culture. Mycelial growth and sporulation of Trichoderma, Aspergillus and Rhizopus were fovorable on the media appended with extracts of rice straws and oyster mushrooms. This results indicate that the rice straw media and mushrooms give favorable conditions for the occurrence of the fungi in the mushroom houses. Mycelial growth of Trichoderma spp. was favorable on saw­dust extraction media and rice bran extraction media, and the spawns inoculated at the mushroom beds present media of the fungi.