The baculum is the bony part of the penis. In the species of the subgenus _Neotamias_ the proximal part of the baculum is termed the shaft, and the distal upturned part is termed the tip. On the dorsal side of the tip there is a longitudinal ridge termed the keel. The proximal end of the shaft is termed the base (see fig. 19). Depending on the species, the shaft varies from 2.11 to 5.28 mm. in length, and the base may or may not be widened or deepened. The purpose of this report is to: (1) Show the usefulness of the structure of the baculum as a taxonomic character in chipmunks; and (2) compare a classification based on the structure of the baculum with a classification based on the structure and appearance of the skull and skin. METHODS, MATERIALS, AND ACKNOWLEDGMENTS The bacula which were borrowed from the University of Michigan, Museum of Zoology, were processed according to the method described by Friley (1947:395-397), whereas all others were processed according to the method described by White (1951:125). Thus the bacula that were borrowed from the University of Michigan, are maintained there in a separate collection, whereas the bacula borrowed from other museums and those that are at the University of Kansas, Museum of Natural History, are housed with the skulls of the corresponding specimens. All measurements of the bacula were made by means of an eyepiece micrometer. A total of 194 bacula were seen. All of these are in the Museum of Natural History of the University of Kansas, unless otherwise indicated by the following symbols: BS United States Biological Surveys Collection. CN Chicago Natural History Museum. LA Los Angeles County Museum. MM University of Michigan, Museum of Zoology. NM United States National Museum. UU University of Utah, Museum of Zoology. I am grateful to Professor E. Raymond Hall for guidance in my study and thank Drs. Robert W. Wilson, Keith R. Kelson, and Edwin C. Galbraith, as well as other friends and associates of the Museum of Natural History, University of Kansas, for encouragement and valuable suggestions. Dr. William L. Jellison, United States Public Health Service, aided me in part of my field work and kindly sent me some specimens of chipmunks. My wife, Alice M. White, made the illustrations and helped me in many ways. For the loan of bacula I thank Dr. William H. Burt, University of Michigan, Museum of Zoology. For permission to search for bacula on study skins, and to process those that were found, I thank Miss Viola S. Schantz, United States Fish and Wildlife Service, Mr. Colin C. Sanborn, Chicago Natural History Museum, Mr. Kenneth E. Stager, Los Angeles County Museum, Dr. David H. Johnson, United States National Museum, and Dr. Stephen D. Durrant, Museum of Zoology, University of Utah.