Trichoderma harzianum is an efficient biocontrol agent that is commercially produced to prevent development of several soil pathogenic fungi. Different mechanisms have been suggested as being responsible for their biocontrol activity, which include competition for space and nutrients, secretion of chitinolytic enzymes, mycoparasitism and production of inhibitory compounds. Nevertheless, the biocontrol activity of T. harzianum could be affected by environmental cues, that include among others, the presence of plant nutrients at the field level. Also, the isolates of Trichoderma harzianum attack the commercial mushroom Agaricus bisporus colonize spawned compost and result in substantial yield reduction. This medium was based on the formulation of harzianum selective medium (THSM). Glucose in the medium serve as a source of energy as well as carbohydrate source and Dipotassium hydrogen phosphate buffers the medium. Magnesium phosphate act as a source of ions and sulphates. Ammonium nitrate provides source of nitrogen. Chloramphenicol suppresses the growth of accompanying bacteria. Rose Bengal is a selective agent that inhibits bacterial growth and restricts the size and height of colonies of more rapidly growing moulds. Care should be taken not to expose this medium to light, since photodegradation of Rose Bengal yields compound that are toxic to fungi.