From 1984 to 1991, 126 males and females were marked and monitored in a wild population of Propithecus verreauxi in southwest Madagascar. Animals were assigned birth years, based on known birth date or degree of dental wear calibrated by recapture data. Twenty-one of 27 groups identified in 1984 persisted within stable home range boundaries, 6 groups disappeared, and 6 new groups formed. Only males transferred between existing groups, although individuals of both sexes left groups to form new groups. The home range of the group into which a male transferred usually overlapped that of his group of origin. On average, 35% of males aged 3–6 years transferred each year. Older males transferred at a lower rate. Three to 6-year-old males were more likely than older males to transfer two or three times. Most 3–6-year old males transferred into groups with a higher proportion and number of females, whereas older males were progressively more likely to transfer into groups with a lower proportion and number of females. New groups were formed by older males, whose mates had died or who had not been successful in joining an existing group, and by young, nulliparous females. Males have rarely been observed for more than 5 years in the same group. Dispersal by 3–6-year-old males appeared to be voluntary. Females helped evict non-natal males from the group but did not prevent new males from joining. Age at first reproduction was 5 years in this population. Forced secondary dispersal may occur when maturing females cease to tolerate males who may be their fathers. © 1993 Wiley-Liss, Inc.