I studied the social organization of the Harris' Hawk (Parabuteo unicintus) in Arizona, 1984-1986. Breeding groups ranged in size from 2-7 and averaged 3.8 hawks. Offspring fledged during previous nesting attempts accounted for 72% of immature helpers. Aggregations averaged 5.9 hawks and were composed primarily of individuals from 2 or 3 neighboring groups. I identified 2 affiliative behaviors and 5 aggressive behaviors that Harris' Hawks used during social interactions. Behaviors that constituted overt aggression were rare in groups, but occurred in aggregations during interactions between hawks from different groups. Groups defended only their nesting areas during nonbreeding periods but defended foraging and nesting areas during breeding. Groups formed aggregations only during nonbreeding periods in specific areas between territories. I observed a peak in aggregation formation about 2.5 weeks before nesting. Open water was used frequently by nesting Harris' Hawks for drinking and bathing. Water sources were not defended and were shared by >1 group.