Ten chimpanzees (Pan troglogytes), aged 18-24 months, housed without mothers as two dyads and two triads, were subjected to social separation. Two issues were addressed: the effects of peer separation in chimpanzees; and differential responses by subjects living in dyads compared with those living in triads. Chimpanzees that were alone during separation reacted with high levels of "protest" alternating with "despair" throughout the separation period. The continued presence of one cagemate, during separation from a third, was a strong mitigating factor. Even when the primary attachment was formed with the absent cagemate, the remaining chimpanzees clung to each other and the levels of protest and despair, when present, were low. Upon reunion, neither "detachment" nor heightened levels of clinging were conspicuous, but there was increased social interaction. The data on separation of chimpanzees are intermediate between those of humans and monkeys separated from mothers or peers. The increased social interactions during reunion, including looking, are comparable to the visual vigilance reported for humans.