Species demonstrating intraspecific variation in social systems can be powerful models for understanding evolution of those systems. As a group, marmots exhibit several types of spacing systems, usually involving some degree of territoriality. Researchers have described populations of 1 species, the woodchuck, Marmota monax, as territorial and as nonterritorial, and such variation has been linked to ecological conditions. I used direct observations of individually recognized animals to describe the spacing system of a high-density population of Marmota monax in southern Maine. This population exhibited intrasexual territoriality. Home range overlap generally was higher between males and females than between same-sex individuals, and woodchucks tended to approach more members of the same sex more quickly compared with members of the opposite sex. Time spent scent marking varied across the active season but did not vary by sex. Males had larger home ranges than females, and home range sizes varied over time, perhaps in response to resources. Amount of overlap also was greater in some years than others, and such changes may be related to kinship. Philopatry and timing of dispersal also vary in this species and have implications for the evolution of sociality.