Green moulds caused by Trichoderma harzianum biotypes Th2 and Th4 are responsible for important economic losses on mushroom farms producing Agaricus bisporus, whereas other biotypes present in farms have only slight effects on the cultivated mushroom. Adaptation of Th2 and non-Th2 strains of Trichoderma to grow in compost prepared for mushroom cultivation was compared through extracellular enzyme activities and competition with bacteria isolated from compost. No specific differences between Th2 and non-Th2 strains were observed for the activities of the 17 secreted enzymes tested. Confrontations of 27 bacterial isolates with 4 Trichoderma strains revealed that the aggressive Th2 strains were affected by a lower number of bacterial isolates than the non-Th2 strains. Capacities of T. harzianum, biotype Th2, to colonise mushroom compost are not due to a specific ability to produce enzymes necessary for compost degradation but to their tolerance to the inhibitory effect of bacteria present in compost.