The casing layer is the material used as top covering of the compost in commercial mushroom cultivation. Here, the ecologic modification that implies the change from the vegetative to the reproductive growth phase takes place, so that it is the medium where the fructification occurs. The complex existent interaction among the different physical, chemical and biological properties of the casing layer hinders the establishment of the optimal conditions that this material should gather. So, the knowledge of the different factors that they participate in the fructification process results of special interest. Among these, next to the own genetic capacity of the mycelium to fructify, physical and environmental factors, chemical and nutritional factors and microbiological factors are included. A revision of microbiological factors involved in Agaricus bisporus fructification is carried out in this work.