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Field observations of 31 pikas, Ochotona princeps, were made during the autumn harvesting season in Colorado. Sizes of home ranges and territories were calculated. Lack of overlap of home ranges was the result of social interactions. Each occupant was thought to possess territorial motivation centering around its hay piles. The narrow boundary between two neighboring territories was well recognized by both occupants. Most invaders will recognize the existence of territories before invasion. Males invaded territories of others more frequently than females. Territorial defense was not successful against dominant invaders. Males were generally dominant to females and adults to juveniles. Body weights of dominants were heavier than those of subordinates. Dominants frequently invaded hay pile areas of subordinates, whereas subordinates either did not invade the territories of dominants or their invasion avoided the hay pile areas of dominants. Pairing situations involving juveniles were accepted. Mutual repulsion between females was apparent, and appeared to be connected with sedentariness. Less favorable habItats were occupied by juveniles. The social organization of O. princeps as compared to three other species of Ochotona is unique in the possession of a territory centered around hay piles. Other basic social relationships are similar. The development of calling parallels the occurrence of harvesting behavior.