Patterns of baculum development were studied in Nyctalus noctula and Vespertilio murinus and found to be similar in both species. A cartilaginous baculum anlage was evident at late embryonic stages. Initially, the distal part of the baculum develops during embryogenesis as a cartilage. Then the growth of the shaft and formation of the proximal portion occurs and it is shortly followed by ossification of the whole structure. The baculum continues to grows during the first months of the postnatal life and its length is correlated with that of the forearm. The baculum reaches its definitive shape and size by the age of 1.5 month in N. noctula and two months in V. murinus. Therefore, after this time period, the shape and size of the baculum cannot be used for age determination.