Introduction. Mixed mycobacterial infections are an urgent problem for phthisiology and pulmonology, which has not been studied enough. The aim of the study was to assess the frequency of detection of mixed populations of mycobacteria in patients with tuberculosis and mycobacteriosis and to determine the species composition of these populations. Materials and methods. Clinical isolates of mycobacteria isolated from 5531 patients who were treated or who sought advice from the FSBI «TSNIIT» were studied. The culture was isolated in the BACTEC MGIT 960 automatic crop growth registration system, species identification was carried out by molecular genetic methods using the GenoType Mycobacterium CM/AS kit (Hain Livescience, Germany) and/or a set based on multiplex PCR in real time. The conclusion about the presence of a mixed population was made if a mixture of two species of mycobacteria was isolated during the entire observation period or the species structure of the population changed dynamically. Descriptive statistics were used to evaluate the results of the study, the chi-squared criterion was used to determine significant differences between groups; differences were considered statistically significant at p < 0.05. The entire analysis was carried out using Microsoft Excel (Microsoft, USA). Results. The frequency of detection of mixed mycobacterial infection by culture method in patients of phthisiological institutions for the period 2011-2023 was 1.16%. The most common combinations of species were M. tuberculosis + M. avium and M. avium + M. intracellulare.