Columbian black-tailed deer (Odocoileus hemionus columbianus) have been live-trapped for various experimental purposes since 1955 at the Hopland Field Station of the University of California. Most of the deer trapped have been tagged, belled and released on the station for studies of movements and population dynamics. In the past, captured deer were aged by visual inspection of the molariform teeth with the aid of a jaw spreader, a flashlight, and a dental mirror. Since the gape of a deer does not extend even to the premolars, this method does not allow a very satisfactory evaluation of tooth wear and has proven too inaccurate for our purposes. At the suggestion of Dr. Carl B. Koford, who worked at the station during 1957 and 1958, a technique was sought for taking dental impressions to provide a permanent record which can be compared with known-age jaws for more accurate aging. Such a technique was developed and has been successfully used on 45 deer during 1958 and 1959. Since recaptures during succeeding years are frequent, a series of impressions can be collected from individual animals. These series will even-