The research was conducted in a mushroom growing facility located in Krasne near Rzeszow, consisting of three production halls. The halls had identical thermal and humidity conditions but different composts (dices 1, dices 2 trademark artificial substrate, conventional substrate), and casings soil (thermally disinfected, chemically disinfected, thermally and chemically disinfected). Fragments of spawn of Agaricus bisporus, compost and casings were put on Petri dishes with acidified PDA nutrient medium. Besides, in relative aseptic conditions, Petri dishes with glucose-potato medium acidified (with 50% lemon acid) and not acidified were prepared. They were placed on upper and middle shelves in four randomly chosen places in each hall. Four Petri dishes with acidified an four with not acidified medium were put in each such point, they were opened and left there for 40 hours, then they were closed and kept in thermostat for 2 weeks. Analyses were carried out at following dates: A – prior to casing application – 16 days after dice and conventional compost were laid, B – during third harvest – 3 weeks after production cycle start. The number and species composition of microorganisms which accompany the mushroom cultivation depended on the healthiness of: the compost, casing and spawn of Agaricus bisporus. The artificial substrates recommended for common mushroom cultivation are not always free from harmful microorganisms and the spawn of Agaricus bisporus which they contain Trichoderma spp may also be paralysed. by the pathogenic species. Precise double disinfecting of the casing – with steam and chemical – eliminates completely Trichoderma spp. Verticillium spp Mycogone perniciosa the species harmful to common mushroom. The presence of competitive and pathogenic fungi in the cultivation halls already at the beginning of the production cycle is a serious thread of the cultivation of common mushroom because their rapid spread shortens the span of fruiting body harvests.

33-41