The chromosomes of the Asian mice, Mus fulvidiventris (boodugal), are typical of the Mus in general, viz., 40 telocentric chromosomes. The centromeric heterochromatin does not fluoresce brightly. The G band pattern of the euchromatin is the same as that of M. musculus. The diploid number of M. dunni is also 40, but each autosome possesses a short, heterochromatic second arm. The X chromosome is a long submetacentric, whose entire short arm and the terminal segment of the long arm are heterochromatic. The Y is a long telocentric and is heterochromatic. The G band pattern of the long arms of M. dunni is again very similar to that of M. musculus. Thus, karyotypic changes in M. dunni involved only the addition of C bands. Mus dunni and M. booduga are sympatric in many localities in India, but they can be separated by karyological and subtle morphological differences.