Thirty-eight bacula of the males of Mus musculus bactrianus and Mus booduga/dunni complex were examined. The M.m. bactrianus baculum has a round base gradually extending into a shaft tipped by a cartilaginous cap which is bifurcated at the distal end. In the M. booduga/dunni complex, the bacular base is squarish or triangular in shape and clearly marked off from the shaft. The cartilaginous cap at the distal end of the shaft is deeply cleft. The baculum of the mice of M. booduga/dunni complex is longer than that of M.m. bactrianus.
