Ascomycetes Fusarium solani and Alternaria solani are major potato pathogens. When it comes to organic farming, methods of protection from the diseases caused by those pathogens are problematic because the allowed direct methods of protection are insufficient (copper formulations, alternative substances used for plant fortification that are based on natural materials and medicinal plants, such as MycoSin®VIN). Many fungi species are used as bio-fungicides, of which Trichoderma species are the best-known. Trichoderma species are known for antagonistic behaviour with mechanisms (such as: competition, antibiosis, mycoparasitism), whose activity surpresses concurrent phytopathogenic species. Even though they are widely used, commercial bio-fungicides based on Trichoderma strains are not registered in Croatia. Antifungal potential of Trichoderma strains widely differs even when it comes to biotypes of the same species. For that reason, in order to recognize promising isolates to be used for the creation of bioformulation, long-term research has to be conducted and laboratory testing is essential. The goal of this thesis is to identify possible antagonism and antibiosis of the autochthonous isolate T. longibrachiatum (STP1) on two phytopathogenic, related fungi, Alternaria solani (AS) and Fusarium solani (FS) - the effect of STP1 on the mycelial growth of AS and FS by measuring the rate of growth of STP1 mycelium in order to evaluate antagonism and antibiosis. In this thesis, possible antagonistic capability of phytopathogenic species F. solani against A. solani and autochthonous isolate (STP1) of T. longibrachiatum was studied. For the purposes of this thesis, the method of dual cultures and its modification was used in order to establish volatile compounds. With the classical method of dual cultures and incubation at 20 °C, isolate STP1 significantly inhibited growth and development of pathogen F. solani (I = 58%), and A. solani (I = 67%), while at temperature of 15 °C, it had considerably weaker suppressive effect. In the study of volatiles and incubation at 15 °C and 20 °C, anthagonistic isolate STP1 showed good inicial effect of volatile compounds on opposing pathogens.