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Territorial defense and maintenance are an important facet of the social ecology of most carnivore species. From January 1991 to June 1993, we observed 54 coyotes (Canis latrans) for 2507 h in Yellowstone National Park, Wyoming, during which we observed 112 instances of territory defense. The identity of the coyotes involved in challenging and evicting intruding animals was known. Alpha coyotes were most likely to be involved in territorial evictions of intruding animals, followed by beta individuals; pups participated little in territory defense. Coyotes evicting intruders generally had a numerical advantage when challenging the intruders. Territory-defense rates were highest during and immediately after the breeding season and during the time of pup emergence from the den. All chases of intruders ended at the boundary of the resident coyote pack’s territory. Physical contact between the intruding animal and resident pack members occurred with no intruders being killed or seriously injured. Physical contact consisted of fighting and ritualized behaviors, with only a few instances in which fighting led to blood being drawn or minor injuries to the intruder. Most encounters resulted in a hasty retreat from the area by the intruder(s). While indirect means of territory maintenance (i.e., howling and scent-marking) were utilized by resident packs, trespassing still occurred and direct confrontation was needed to enforce boundaries and assert occupancy against other resident packs, as well as against lone intruders seeking territory vacancies. Territoriality was advantageous in that territory holders generally had higher survival rates (except pups), contributed all the reproductive effort to the population, and had greater access to food resources. In contrast, non-territorial animals had lower survival rates, did not breed, and had reduced access to food (ungulate carcasses) during winter.