We examined morphological variability and evolution of the baculum (os penis) across the Mustelidae through allometric analyses and character mapping. Fifty-four species and 26 genera (including 2 fossil forms) were examined with numerous caniform out-group species. Allometric analyses showed that bacular length is relatively constant across mustelids and caniforms; only a tendency to a slightly shortened baculum in mephitines was observed. Character mapping revealed the ancestral mustelid baculum to be an elongated rod-shaped bone that lacks a urethral groove and possesses a simple, nondistinct distal tip. This form is largely retained in mephitines and, to a lesser degree, in lutrines. From the ancestral condition, it is possible to derive forms with a more complicated head that has projections and openings (e.g., melines, Eira barbara, Galictis, Gulo gulo, Martes) or spoon-shaped and cup-shaped processes (e.g., Ictonyx, Mellivora capensis). Another evolutionary trajectory involves the distal tip of the baculum becoming hook-shaped and the urethral groove well developed (e.g., Mustela, Vormela peregusna). Although the structure of the baculum distinguishes closely related species, many features are derived independently in more distantly related forms. Therefore, bacular structure provides restricted phylogenetic information and should be analyzed in concert with other data sources (e.g., morphology of the basicranial region).