The relationships among dominance, age and aggressive behaviour of marked, female pronghorn (Antilocapra americana; Family Antilocapridae) were studied in north central Colorado. Females formed dominance hierarchies with few circular relationships. Unlike other female ungulates, dominance rank was not correlated with age. Dominance rank was negatively correlated with rate of aggression initiated in summer and winter; however, aggressiveness as an individual trait was not related to dominance. Older females received higher rates of aggression than younger females in summer and winter. During the rut, dominance rank was correlated with rate of aggression received but not with rate of aggression initiated. Patterns of aggression that differ by season within a given hierarchy of dominance relationships suggest a different cause or function of aggressive behaviour in different seasons.