Intensive studies of the woodchuck were carried out on the two farm areas of the Patuxent Research Refuge, Laurel, Maryland, from 1947 to 1949. Extensive studies were made in several localities throughout Maryland. A technique found highly successful in obtaining live animals involved the use of dug-in traps, placed so that woodchucks in the burrow system had no choice but to enter the trap. In handling animals secured in live-traps, a cotton mesh fish bag was found to be more practical than wire cones or burlap sacks. The species Marmota monax is presently divided into nine subspecies, distributed over most of the United States and southern Canada. The southern woodchuck is distributed through the middle eastern United States, with the largest populations occurring in the Middle Atlantic States. An extension of the range into the eastern shore section of Maryland has occurred within the past 40 years. The greatest longevity record was ten years. Animals four years of age have been handled at the Rose Lake Wildlife Experiment Station in Michigan and at the Refuge. The natural life span is from five to six years. Hibernation is progressive, the older and fatter animals going in first, followed by the yearlings and juveniles. The same order of precedence holds true as the animals emerge, the larger ones appearing first. A hibernating captive animal was studied under simulated outdoor natural conditions in 1948-49. Weight loss of a number of animals during hibernation was 30.45 percent and continued for several weeks after emergence. Length of gestation periods were obtained. Two captive females were mated with a single male, resulting in gestation periods of 32 days (plus or minus one day) and 33 days (plus or minus one day). An artificially inseminated female produced a litter in 31 or 32 days. An average litter of 4.6 young were recorded from 29 females examined. Weights proved to be an accurate and practical field method of aging woodchucks. Over 500 weights were obtained during the investigation.The sex ratio among 215 animals was 1.03 males to 1 female. The average fall population on the Check Farm was 12 animals per 100 acres, that of the Conservation Farm about 18 per 100 acres. Indications are that the southern woodchuck is increasing throughout most of its range. Food habits studies in the spring and summer showed that red clover, white clover, grass, chickweed, and alfalfa were the foods eaten most often and in the largest amounts. Activities of man and predators were the most important mortality factors. Flooding, disease, and parasites were found to be of negligible importance. Den construction varied according to land use and site. The winter dens located in wooded areas usually had only one entrance, rarely more than two. Summer field dens were characterized by a main entrance, with a mound of fresh soil and a smaller plunge hole, sometimes well concealed in the surrounding vegetation. Excavations conducted during the investigation further substantiated the theory that woodchucks hibernate in plugged off chambers. A total of 22 dens were excavated and only one animal recovered. Woodchucks were widely selective in the choice of soil types but showed a preference for the sandy loams. Well drained and protected situations were the primary factors in selection of winter den sites. In the winter, the use of woodchuck burrows by rabbits was light wheal winds were moderate and temperatures above 30⚬ F. Moderate use of holes was noted when the temperature varied between 20 and 30⚬ F. Heavy use was observed when strong winds were blowing and temperatures were below 20⚬ F. Other animals known to frequent dens, were skunks, opossums, foxes, raccoons, weasels, wood mice, gray squirrels, jumping mice, chipmunks, meadow voles, shrews, house mice, pine voles, and several species of snakes. Home ranges were directly dependent upon the location of dens in relation to the habitat. Animals inhabiting dens in ravines, woodlots, and odd areas were often known to travel several hundred yards to feeding areas. Woodchucks in clover and alfalfa fields sometimes restricted their home ranges to an area within 20 yams of the den. Seasonal movements were noted in the spring shortly after emergence from hibernation, in the summer as the young dispersed from the home den, and again in the fall when the animals moved from the summer dens to hibernating areas. Woodchucks exhibited territoriality within the home den.