The baculum in microtines consists of an elongate stalk, having a laterally, and to a lesser extent dorsoventrally, expanded base and an attenuate distal shaft. Usually, three digitate processes of cartilaginous material in which additional ossifications may occur arise from the terminus of the shaft. The proportions and curvature of the stalk vary as do the proportions of the terminal ossifications to each other and to the stalk. In some species one or more of the digital processes are frequently completely unossified. [Illustration: FIGURE 1. The baculum in _Microtus ochrogaster_--orientation and variation with age. _a._ Diagram of a sagittal section of the posterior half of a vole, natural size. The penis, containing the baculum (in black), extends ventrally from a point posterior to the pubic symphysis (stippled), along the body wall, and bends posteriorly at the distal end. _b._ Distal end of penis (× 2) showing baculum (in black), the urethra (solid lines) adjacent to the baculum, and the corpus cavernosum (broken lines) proximal to the baculum. _c._ Oblique view of the cross section of penis (× 4) shown in Figure 1 _b_. The thick-walled urethra lies ventral to the curved corpus cavernosum. A thinner-walled blood-vessel lies dorsal to the corpus cavernosum. The anatomically ventral side of the baculum, in the normal non-erect penis shown, is seen to face dorsally. _d._ Graph showing the relationship between size of baculum, size of animal, and development of digital ossifications. Circles show presence of ossification in stalk only; circles enclosing dots indicate presence of secondary ossification in median process also; large dots indicate the addition of tertiary ossification in one or both of the lateral digitate processes.] Preserved specimens of _Microtus arvalis_, _Microtus agrestis_, _Microtus orcadensis_, _Microtus nivalis_, _Microtus guentheri_, _Microtus subterraneus_, _Clethrionomys glareolus_, and _Ellobius lutescens_ were provided by Prof. Robert Matthey of Lausanne, Switzerland. J. Knox Jones, Jr. carefully saved the bacula with specimens of _Microtus fortis_ and _Clethrionomys rufocanus_ from Korea. Dr. W. B. Quay, Department of Zoology, University of California, supplied specimens of _Synaptomys cooperi_, _Phenacomys intermedius_, and _Microtus oregoni_. Dr. Franklin Sturges and Mr. John W. Goertz, Museum of Natural History, Oregon State College, Corvallis, have provided specimens including bacula of _Clethrionomys occidentalis_, _Microtus oregoni_, and _Microtus townsendii_. Dr. Randolph L. Peterson and Mr. Bristol Foster, Royal Ontario Museum of Zoology, Toronto, Canada, provided specimens of _Phenacomys intermedius_. Dr. J. N. Layne, University of Florida, Gainsville, Florida, presented me with a baculum of _Microtus parvulus_.