Abstract <p>Among the number of stress factors affecting mammals in urban environments, a high parasite load plays an important role. The resistance of a population to this factor can be assessed by the allelic diversity of certain genes, for example, those of the major histocompatibility complex (MHC), which play a key role in immune defense. We analyzed the allelic diversity of exon 2 of the DRB gene in striped field mouse populations in four parks in Moscow. Using amplicon sequencing of the target fragment on the Illumina NovaSeq 6000 platform, 27 alleles were discovered, nine of which were common to those known for the bank vole. The largest number of alleles, including unique ones, were noted in the least urbanized of the studied areas with a multispecies community of small mammals (Bitsevskii Park). In addition, the greatest diversity of individual genotypes and a relatively smaller number of alleles in the individual’s genotype were observed here. In the three other parks located in areas with a higher degree of urbanization, the number of alleles represented in the population and the diversity of individual genotypes were smaller, but the number of alleles represented in the genotype of one individual was higher. In the most urbanized area, in the absence of other small mammal species in the community (Neskuchnyi Garden), the absence of neutral variability was noted—each of the alleles present in the population encoded a unique amino acid sequence with an inherent variant of the antigen-binding site. It is assumed that these differences reflect the ways of adaptation depending on the degree of anthropogenic pressure. An assessment of the similarity of populations based on the presence of common alleles showed greater similarity in pairs from the right (Neskuchnyi Garden and Bitsevskii Park) and from the left (Terletskii Park and the Main Botanical Garden) banks of the Moskva River, which may reflect the historical connection of these territories.</p>