The cotton-topped, tamarin, Saguinus oedipus oedipus, is a species that is severely threatened in the wild. Thus, if its survival is to be ensured, a self-sustaining captive population needs to be established. However, the breeding results of cotton-topped tamarins in zoos are far from optimal; infant mortality is high and the fertility of captive-born specimens is critically low. The roots of both these problems may be sought in the social behavior of this species in zoos. In order to evaluate the influence of the zoo environment on behavior, a comparison is made between the behavior of a group of tamarins housed in a typical zoo environment and groups of successfully breeding cotton-topped tamarins housed off public display in the zoo. Significant differences were found between the behavior of the animals in the two different situations. Behavioral differences were correlated with the number of visitors in the zoo environment and with the design of the cage.