JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Society of Mammalogists is collaborating with JSTOR to digitize, preserve and extend access to Journal of Mammalogy.Outside of the laboratory, where it has been rather intensively studied, the opossum (Didelphis virginiana) has been little investigated. In many states throughout its range, the opossum ranks as an important fur species and in some it is valued for its meat. Though the price of the meat or fur of an individual opossum usually is low, collectively the annual opossum crop is of considerable economic significance.Despite its importance, scarcely any attempts have been made to conduct state-wide surveys to serve as guides in the management of opossum populations. This probably is due in part to a lack of suitable techniques for collecting data bearing on the species. Sex ratios and numbers of young per adult female are important indicators of productivity, and under some conditions these ratios also may be used to measure the sizes of animal populations and the extent of fluctuations therein (Kelker, 1945;Petrides, 1949). The full extent of their usefulness in studying opossum populations has not been explored.The present study was undertaken primarily to determine methods for rapidly determining the sex and age of opossums particularly during the trapping season. Data reported here which are applicable to other seasons were collected incidental to studies of other species and are not as detailed for these periods as could be desired. Information is provided, however, which would seem to be useful in aiding future intensive field investigations into the life history and ecology of the opossum as well as in assisting in more extensive surveys. Since during the breeding season the capture of a female usually also results in the capture of a litter, the opossum lends itself unusually well to studies of reproduction. Data are provided permitting the determination of the age of immature opossums, thus establishing a basis for estimating the dates of reproductive events.Acknowledgments.-This study, a part of my doctoral dissertation, was com-, undergraduate assistants, all helped from time to time in various phases of the study. The Ohio Division of Conservation and Natural Resources and Commissioner H. A. Rider, Executive Director of this organization, deserve recognition for their support of the project as a portion of the program of the Ohio Cooperative Wildlife Research Unit. Messrs. Joseph Schwartz, owner, and Edward Frazier, skinner, Columbus Hide and Fur Company, Columbus, Ohio, generously permitted the examination of bought pelts and carcasses. Mr. Frazier also helped greatly by saving certain specimen materials.
