Primary colonization and the early growth of the mycelium of Agaricus bisporus in compost could be improved to increase the rate of substrate utilization. The production of enzyme activities during the vegetative growth of a commercial strain in compost were determined. N-acetyl-glucosaminidase activity peaked during the first week whereas the activities of polysaccharidases decreased with the replacement of autochthonous micro-organisms by A. bisporus. A low level of water soluble sugars was maintained in compost inoculated with A. bisporus in comparison with a non-inoculated compost. Microbial residues and hemicelluloses appeared to be important nutrient sources during the early growth.