In this study was quantified bacular size, variation, and allometry in mound-building mice (Mus spicilegus) from the western Carpathians (Slovakia). The study presents the first knowledge of morphological variation in three baculum traits based on the analysis of 11 adult and 60 subadult males. According to descriptive statistics, bacula are variable in size. Despite the small number of adult individuals, the Mann-Whitney test indicated significant differences (p < 0.05) in proportions between age groups. However, to confirm these results, larger numbers of adults and subadults will need to be tested in the future.