(I) A colony of black-tailed prairie dogs in the Philadelphia zoo is being used for long term studies of various aspects of social behavior including pattern of social organization, stylized means of communicating, and the effects of individual behavioral differences on communication and social structure. (2) This report deals with the annual cycle of social behavior manifested by the colony during the first three years of study. In gross outline an annual cycle can be described as follows: during the summer most individuals have "friendly" inter-relationships, and little organization is apparent. During autumn the animals gradually band into coterie groups and each group begins to defend a common territory, primarily through a highly ritualized procedure of "challenging" neighbors across a common boundary. By midwinter the adult males are very aggressive. With the advent of the winter breeding season, the females too become aggressive, and some establish territories that may subdivide coterie territories. In spring males and then females gradually become less aggressive, so that by about the time the pups emerge onto the surface the relaxed social relationships typical of summer are developing generally. (3) The shifting pattern of social organization is based on the division of individuals into classes on the basis of sex, age, dominance, and coterie and other bonds, and modified by the considerable inter-individual differencees in what may loosely be called "temperament." (4) Comparisons of the patterns of social organization shown by the zoo prairie dogs with those thus far described in the literature for wild populations show fundamental similarities, although the very high density and lack of opportunity for emigration in the zoo colony have lead to some modifications which are described and discussed. (5) The complexity of the social; organization of the black-tailed prairie dog is unusual for a rodent, and a model is proposed to account for its evolution from more widespread patterns of rodent social behavior. As the model emphasizes the role of the very open grassland habitat of the species in intensifying the natural selection potentially responsible for the evolution of this socal complexity, it suggests that habitat similarities may underlie evolutionary convergences in the social systems of black-tailed prairie dogs and savanna-dwelling primates.