Abstract <p>Karyotype mosaicism in the number of B chromosomes (Bs) was studied in the Korean field mouse <i>Apodemus</i> <i>peninsulae</i> from Zabaikalsky krai. High frequency of mosaics was revealed, especially in samples from the “southeastern” group II (91%) from the right banks of the Shilka and Onon rivers. Mosaics had from two to six cellular clones. Animals with 2, 3, and 4 cellular clones were predominant. For the first time, the mitotic instability index (<i>MI</i>) was analyzed in the Korean field mouse. Approximately 70% of animals had high <i>MI</i> values along with negative accumulation index (<i>A</i>) values. These quantitative mosaicism parameters indicate mitotic instability of B chromosomes and the prevalence of their elimination processes over accumulation. It is demonstrated that frequency characteristics of mosaicism (in addition to the Bs quantitative and morphotypic parameters) can be used as another trait for differentiating the “northwestern” and “southeastern” geographic groups of samples, the boundary between which runs along the Shilka and Onon rivers. The adaptive value of mosaicism in <i>A.</i> <i>peninsulae</i> populations is discussed.</p>