This research was carried out to determine the interaction between some bacteria naturally existing in casing soil and Agaricus bisporus (Sylvan Hauser A15) hypha in laboratory (in vitro) and cultivation (in vivo) conditions, and to confirm its effects on mushroom yield. Totally 32 bacteria (3 Gram (+) and 29 Fluorescent Pseudomonads) was isolated from casing soil and healthy sporophores. As a result of in vitro experiment carried out to determine the effects of bacteria on mycelium growth of A. bisporus, 24 bacterial isolates were found more effective at the rate of 2 to 115% than control treatment. To determine the effects of bacterium, chosen at the end of in vitro experiments, on mushroom yield in cultivation conditions, three experiments were established in March, May and July in 2008. At the end of experiments, bacterial isolates provided 8 – 40 % increase in total yield. Population density and change in population number related to time was observed during growing period, after the inoculation of bacterial isolates into casing soil. According to the results, Pseudomonas fluorescens (T 4/2 and Ş 8), P.putida (Ş 2/1 and Ş 10) and Bacillus mycoides (T 7/2) bacterial isolates were colonized successfully both in casing soil and sporophores.