Eight monkey mother-infant pairs were separated for a period of two weeks and measurements were made of the behavior of the infants before, during, and after reunion with their mothers. All infants showed emotional disturbance in response to separation with drastic increases in crying and decreases in playing. From an initial response of violent protest, they passed into a stage characterized by low activity, little or no play, and occasional crying. This pattern of response to separation in monkey infants is similar to that described by Bowlby in human infants. Of Bowlby's three phases—protest, despair, and detachment—the authors find striking support in the monkey, particularly with respect to the first two.