Every part of the Reservation is used by opossums, but their activity is concentrated in the woodland, and all dens found were in woodland. Most parts of the fields are within 100 yards of the edge of the woodland and no point is more than 700 feet from the edge. Most of the opossums' foraging in fields was concentrated along the edge; otherwise they tended to follow creeks and gullies and they follow well worn trails more often than they do in the woods. Within the woodland, activity tended to be concentrated along the small streams, and along the rock ledges where den sites were plentiful. Throughout the annual cycle, and from year to year, there were minor shifts in areas of concentrated activity depending on seasonal changes in food sources such as thickets of wild plum, crabapple, blackberry and grape, with fruits ripening at slightly different times of year. The areas adjoining the Reservation offer somewhat similar habitat conditions, part woodland, part pasture land and some cultivated fields with corn or other crops which provide food sources for the opossum. Under original conditions the area that is now the Reservation probably was marginal habitat for opossums, consisting mainly of open grassland with trees in small and scattered clumps, if indeed they were present at all. There has been steady encroachment of shrubs and trees, originally chiefly confined to near-by bottomlands such as those of the Kaw and Wakarusa valleys. Concurrently, the original hardwood forest of the bottomlands has mostly disappeared, and the land has been taken over for intensive agricultural use. The new upland forest provides a habitat different in many respects from the original bottomland forest. The species composition, in trees and other plants, is somewhat different, with more xeric types, especially on steep south slopes. Logs and large old hollow trees are scarce. The lack of such potential den sites is compensated for by the abundance of holes and crevices along hilltop rock ledges. BEHAVIOR