In vitroevaluation of the antagonistic properties ofTrichodermaspp. againstPyrenochaeta lycopersiciandPhomopsis sclerotioideswas carried out, using two different methods: the cellophane‐agar plate method and the duaí culture method. With these methods, the direct interaction between pathogen and antagonist was demonstrated, showing very remarkable differences between the different isolates. Growth inhibition ranging from 20 to 100% was found. Proliferation in non‐sterilized soil was investigated for the isolates showing the most interesting antagonistic properties. A Belgian isolate ofTrichoderma hamatumwas found to be able to proliferate very well in soil and to maintain a high population density during the whole experimental pe