Improved strains of Trichoderma spp. have been obtained as agents for the biological control of phytopathogenic fungi and nematodes, which strains are capable of overproducing a proteinase, by using a transformation process which involves the introduction of the gene prb1 of Trichoderma which codes for the proteinase PrB1 under the control of the appropriate elements of regulation of the expression, in multiple copies and in a stable way, whereby the control of the disease caused by pathogenic fungi or by netamodes is superior in the transgenic strain than in the wild strain used as receptor of the genetic information.