Abstract 10.1002/zoo.1430130408.abs Masturbation was observed in 9 of 52 captive adult Hylobates (gibbons) over a period of 17 years at the International Center for Gibbon Studies (ICGS). Four females and 5 males masturbated with varying degrees of regularity. The behavior may be related to a disruption in early rearing experiences of Hylobates. This is the first report on masturbation in these animals and documents that this behavior occurs infrequently in captive Hylobates. © 1994 Wiley-Liss, Inc.