In each of three different winters, the largest age group in the population of opossums was that of the newly matured young born in early spring. The old adults were the next most numerous group, and the second-litter young born in early summer were the least numerous. The figures obtained from live-trapping indicate an annual population turnover of approximately 95 per cent, with some 70 per cent eliminated by various mortality factors and replaced by young, the remaining 25 per cent shifting to new areas, with compensatory shifts of individuals replacing them. The various mortality factors which regulate the numbers of opossums are not well known, and even less is known regarding the relative importance of the factors. Food supply and weather are obviously of major importance and closely interrelated in their effect on the population. One large adult opossum that was trapped seemed to be dying from disease and was scarcely able to stand; but others caught near-by before and after were unaffected. The horned owl is perhaps the most important natural enemy of the opossum on the Reservation, and instances of owl predation on opossums were noted. LITERATURE CITED FITCH, H. S. 1950. A new style live-trap for small mammals. Jour. Mamm., 31:364-365. 1952. The University of Kansas Natural History Reservation. Univ. Kansas Mus. Nat. Hist., Misc. Publ., 4:1-38, 4 pls. HALL, E. R., and KELSON, K. R. 1952. Comments on the taxonomy and geographic distribution of some North American marsupials, insectivores and carnivores. Univ. Kansas Publ., Mus. Nat. Hist., 5:319-341. HARTMAN, C. G. 1923. Breeding habits, development and birth of the opossum. Smithsonian Report 1921:347-363. 1928. The breeding season of the opossum (_Didelphis virginiana_) and the rate of intrauterine and postnatal development. Jour. Morph. and Physiol., 46:143-215. 1952. Possums. Univ. of Texas Press, Austin. xvi + 174 pp. LAY, D. W. 1942. Ecology of the opossum in eastern Texas. Jour. Mamm., 23:147-159. LEONARD, A. B., and GOBLE, R. C. 1952. Mollusca of the University of Kansas Natural History Reservation. Univ. Kansas Sci. Bull., 34:1013-1055. REYNOLDS, H. C. 1945. Some aspects of the life history and ecology of the opossum in central Missouri. Jour. Mamm., 26:361-379. SANDIDGE, L. L. 1953. Food and dens of the opossum (_Didelphis virginiana_) in northeastern Kansas. Trans. Kansas Acad. Sci., 59:97-106. WISEMAN, G. L., and HENDRICKSON, G. O. 1950. Notes on the life history and ecology of the opossum in southeast Iowa. Jour. Mamm., 31:331-337. _Transmitted May 4, 1953._ * * * * * Transcriber's Notes