Abstract Agonistic behavior of Gila Woodpeckers, including vocalizations, visual displays, and other related behaviors, is described. Interactions with both con- and heterospecifics were analyzed by stochastic processes, and it is shown that the timing of aggression toward a species coincided with the time during which that species was searching for nest sites or cavities. The behavior shown toward Flickers and Starlings appears to fulfill criteria for interspecific territoriality, and evidence is presented to support the contention that the function of this behavior is protection of nest sites and nest holes from competitors. The effects of intra- and interspecific territoriality on the distribution of the species involved were found to be significantly different. Simultaneous protection of different sized territories is discussed.