Root exudates play an important role in initiation processes of plant pathogen interactions. Therefore, 3 different Trichoderma strains (Trichoderma viride strain RE 1-3-4, T. harzianum strain T39 and T. atroviride strain P1) were screened for their effects on root exudation pattern of tomato, plant symbionts (arbuscular mycorrhizal fungi (AMF)) and 3 selected tomato pathogens (Fusarium oxysporum f.sp. lycopersici (Fol)), Pyrenochaeta lycopersici and Verticillium dahliae). In order to test the effects of Trichoderma on Fol and P. lycopersici via the alteration of the root exudation pattern, a biotest with a microplate reader was established. This technique was not adequate to measure the growth of P. lycopersici, but of Fol. In the microplate reader assay the fungal growth of Fol in the exudates of the tomato cultures inoculated with Trichoderma spp. as well as in the cultures inoculated with Trichoderma and/or AMF clear indications are given that Trichoderma spp. and AMF, respectively, alter the exudation pattern of tomato plants and that these alterations affect the fungal growth of Fol. However, the picture is not clear yet as depending on the tested Trichoderma strain, the used substrate and the time of inoculation, inhibitory as well as stimulatory effects on the fungal growth of Fol were observed. In the presence of root exudates from mycorrhizal tomato plants the fungal growth of Fol was reduced. The inhibitory effect on fungal growth of Fol when the different Trichoderma strains were applied in combination with the AMF were similar as with AMF alone, only for one treatment (AMF + T. harzianum T39) the fungal growth was even lower than for AMF alone. Trichoderma spp. did not have any adverse effects on the degree of root colonisation by AMF. In the dual cultures the growth areas of Fol and P. lycopersici were significantly reduced by the 3 applied Trichoderma strains, whereas, no effects on the growth area of V. dahliae could be detected.