We studied an occurrence of chromosomal aneuploidy for ten individuals in the small Japanese field mouse, Apodemus argenteus, from the western mountainous area of Kanagawa Prefecture, Honshu, Japan. Eight of the individuals examined showed the standard 2n = 46 and FNa = 48 constitutions as the most frequent karyotype. However, of those eight individuals, seven carried an inter-cellular mosaicism with a few metaphases carrying a small-sized supernumerary element. In addition, one individual showed an aneuploid complement that included a small-sized bi-armed B chromosome (2n = 47). The B chromosome consisted of the C-heterochromatin and was characterized by delayed fluorescence by the quinacrine mustard staining, suggesting that its feature is identical to the B chromosomes previously confirmed in other localities. Moreover, an XO female showing a standard autosomal complement with only one X chromosome (2n = 45, XO) was also recognized. According to the present observations, these chromosomal aneuploidies are retained in wild populations of A. argenteus as constantly occurring phenomena.