The objective of the in vitro studies was to identify a Trichoderma strain with a high antagonistic potential against the basidiomycetes Ganoderma adspersum, Ganoderma lipsiense, Inonotus hispidus, Polyporus squamosus and the ascomycete Kretzschmaria deusta. For this purpose dual culture and interaction tests in wood blocks as well as investigations on fungal growth and germination behavior of conidia under different conditions were performed. Hyphal interactions were observed by scanning electron microscopy (SEM). The effect of Trichoderma spp. on wood colonization and degradation of wood decay fungi were quantitatively analyzed by means of dry weight loss measurements of wood and qualitatively by histological studies. The different Trichoderma species all showed an antagonistic potential against wood decay fungi in the in vitro studies. However, significant differences between the species and strains were found (P<0.001). Trichoderma atroviride (T-15603.1) showed the highest competitive activity against most wood decay fungi. An influence of physical and chemical parameters, in particular temperature and water potential on growth and germination behavior of conidia was evident. The species of wood decay fungi showed significant differences in their sensitivity when challenged by Trichoderma. Polyporus squamosus showed an extensive resistance in most laboratory tests indicating that target specificity of the antagonist needs consideration.