This work presents results on the effect of manganese ions on the development and biological activities of the fungal antagonists Trichoderma harzianum, Trichoderma pseudokoningii and Trichoderma viride. The effects of manganese on linear growth of Trichoderma spp. depend on the metal concentration and the fungal species. T. harzianum proved the most sensitive fungus. On the other hand, none of the applied manganese ion concentrations inhibited mycelial growth of T. viride. At the highest concentration of 800 ppm, manganese ions caused a weakening of the conidial germination of T. harzianum and T. viride. An increase in manganese ion concentrations had a negative influence on the effect of T. harzianum on Botrytis cinerea and Rhizoctonia solani.