[Illustration: FIGURE 4. Quarter-mile-square areas on Reservation showing dates and successive sites of capture of individual opossums; (A) subadult male; (B) subadult male; (C) subadult male; (D) adult female. Arrows from circles show courses that were taken by released opossums that were followed; crosses show location of dens to which they were traced.] Evidence of the existence and extent of home range was obtained for those opossums that were trapped on several or many occasions. Records of each were usually well scattered over an area hundreds of feet in diameter. Limits of home ranges are not sharply defined and at any time the opossum may extend its range into new areas. It may shift to a new den from which areas beyond its original home range are readily accessible, and may then occupy a new home range overlapping part of the old one. Or, it may make a relatively long shift, to an area entirely distinct from the original home range and well separated from it. That such shifts are frequent was indicated by the brief span of records for most of the opossums live-trapped on the Reservation. After the first capture and marking an individual was often caught consistently over periods of weeks, only to drop out suddenly either having been eliminated or having moved elsewhere. Of the 111 opossums marked and released, 62 were caught only once and 25 others were recaptured only within a period of one or two months. Relatively few, only 24 (14 males and 10 females), had records extending over more than two months. Many of the opossums trapped were probably at or near the edges of their home ranges which barely overlapped the study area; consequently the chances of recapturing them were poor. Those caught well within the trapping area were much more likely to be recaptured.