The ability of Trichoderma harzianum and T. viride iprodione-resistant mutants to colonize cucumber root system was examined and their effect on plant biomass production was assessed, as well as their efficiency in pathogen control when ap-plied alone or combined with fungicides (iprodione or propamocarb). The dosages which inhibited the radial growth of tested mutants by 50% (ID 50) ranged from 26 to 821 mg/ml of iprodione and from 389 to 1824 mg/ml of propamocarb hydrochloride. For mutant types, the calculated ID 50 values were 90–40 times higher for iprodione and propamocarb hydrochloride, respectively, than for wild types. After introduction of Trichoderma spp. isolates into the soil substrate an increase in dry biomass production of cucumber plants occurred. Plant growth stimulation was associated with improved colonization of the root systems by Trichoderma iso-lates. The best colonizers occupied the root systems in the range from 227 to 1159 cfu per 1 g. They also stimulated their growth by 41–106%. A synergistic effect was noticed in the case of reduction of the number of cfu of Sclerotinia sclerotiorum and Fusarium culmorum on roots after combined application of iprodione or propamo-carb with tested iprodione-resistant mutants.