The early ossification of the baculum noted in the rat and the hamster occurs in _Microtus_ also. A specimen of _Microtus montanus fusus_ (76831, from 5 mi. N, 26 mi. W Saguache, 9600 ft., Saguache County, Colorado) only 74 mm. in total length and weighing only 6.6 grams, had a slender ossified baculum having enlarged ends. This vole was one-half of the average length and less than one-fifth of the average weight of an adult, and of approximately the size at which weaning takes place. The development of the baculum in _Microtus ochrogaster_ was studied in 32 specimens of various ages. The specimens (between Nos. 74994 and 75074) were collected between August 15 and September 4, 1957, at localities on the Great Plains. These specimens were from breeding populations, as evidenced by pregnancy of females and by large size of testes of males. The length and width of the stalk of the baculum, the presence of digital ossifications, the total length of the animal, and the size of the testes were noted. Variability in length of testes is greatest when voles are from 140 to 150 mm. in total length. Sexual maturity is reached rather abruptly when the total length of most individuals is 140 to 150 millimeters. If the baculum likewise underwent more rapid growth at the onset of sexual maturity, greater variability should be evident in the length of the baculum of voles 140 to 150 mm. in total length than in bacula of voles of other sizes. This was the case (see Figure 1d). The baculum does not, however, suddenly reach its maximum maturity.