The dominance relationships and associated agonistic postures of Canada geese were studied during the winters 1963-64 and 1964-65 by observation of the behavior of radio-and color-marked families and individuals living in a large, wild flock in southern Illinois. Part I I. Behavior was not significantly altered by color-marking and attachment of trausmitters. 2. Hostile encounters occurred during almost all activities. They varied widely in frequency and intensity and were especially associated with feeding. 3. Success in hostile encounters was directly related to family size, i.e., large family> smaller family > pair > single. 4. Unified action by all members of a family occurred in 8.5 percent of victories and T5 percent of defeats. 5. All members of a family shared equal dominance status but the success of a family in the rank order was most dependent upon the gander. 6. Only once in 26 fights between unmarked family ganders did the gander of the largest family lose. 7. Dominance position of family individuals decreased immediately upon separation and increased upon reunification of family members. 8. Large families were engaged in significantly more conflicts per unit time than were singles, pairs, and small families. 9. Exceptions to the usual dominance hierarchy occurred after pairs were newly formed. The gander of a newly formed pair could dominate family ganders. 10. Intrafamily aggression was rare and of low intensity. 11. Fights rarely occurred; threats and chases were common. 12. In some instances, rank orders based upon individual recognition could exist. However, stable rank orders in most large flocks appear to be based on recognition of different postures and levels of intensity of threat. 13. The dominance order of geese yields benefit in terms of food and space acquisition and freedom from defeat in aggressive encounters for the pairs and their young in direct relation to those most successful at raising a brood. Part II 1. Postures associated with attack or fleeing or simultaneous tendencies to do both are described. These include actual fleeing or attack, Submissive attitude, Erect, Head-pumping, Rolling, Bent-neck, and Forward postures. 2. The Submissive attitude is exhibited mostly by single geese and probably results from the conflicting tendency to approach (but not attack) and flee from other geese at the same time. This posture functions to identify single geese, allow approach, habituation, and ultimately pair formation, and inhibits violent attack. 3. The Erect posture may take either the form of intention movements of escape or attack and represents an ambivalent motivation between these two tendencies. 4. Head-pumping contains alternating intention movements of attacking and fleeing and represents almost a perfect balance between these two tendencies but is of higher intensity and ritualization than the Erect position. 5. Rolling is a complex portion of the Triumph Ceremony but also serves as the most intense threat of Canada geese and is highly ritualized. The spatial relationships of a gander to his mate and family appear most important in motivating Rolling. Intrusion of another high ranking gander or family on those boundaries may result in violent attack. 6. Erect, Head-pumping, and Rolling serve as three different intensity threats which are recognized by other geese and serve to maintain and establish the rank order of geese without undue fighting. 7. Bent-neck and Forward postures may occasionally represent conflicting attack and flee tendencies but often appear to represent a conflict of attack and remain doing another activity such as feeding or preening. These postures serve to maintain and reinforce a rank order but are probably not very important in initial establishment of rank.