In the juvenile muskrat 5–8 months of age the distal end or digital processes of the baculum are cartilagenous and the calcification within has only just begun. Ossification centers are 3–6 in number. Calcification is nearly complete in all adult animals. The shaft in adults is heavier and more rugose. These differences provide a new technique for age classification of the harvest. Dimensions such as width and length of the shaft are not reliable criteria of age because considerable overlap is found in large samples. Dentition is not a valid criterion of age in Missouri muskrats.