Chemicalization of agriculture increases the resistance of pests to pesticides. Their aggressiveness is enhanced on all crops, including the walnut culture, an example could be the presence of the pathogen Alternaria alternata (Fr.) Keissl. To reduce the resistance and harmfulness of plant pathogens, it is important to use biological methods. According to the researchers, Bacillus spp. are among the most important biocontrol agents against fungal phytopathogens. The purpose is to determine the biological activity of bacteria Bacillus subtilis CNMN-BB-06 towards Alternaria alternata. The object of research was the bacterium strain Bacillus subtilis CNMN-BB-06. Antagonistic activity against A. alternata was studied by method of dual culture on agar nutrient medium. A lawn of bacilli was obtained by seeding a suspension of a 3-day culture onto an agar plate. There was installed at the same time an agar block with grown culture of A. alternata. The index of pathogen inhibition by the bacterium B. subtilis was determined on the 3rd, 7th and 10th days (%). As a result of the study, it was noted that the bacterial lawn significantly inhibited the growth of the pathogen and on the third day the inhibition rate was 33.3%. On the seventh day of the experiment, the bacterium inhibited the growth of the pathogen by 69.6%, on the 10th day - by 70.2%, compared to the control.