Abstract:We observed the social play behavior of individual Yunnan snub-nosed monkeys (Rhinopithecus bieti)of 0 - 3 years of age at Xiangguqing in Baimaxueshan National Nature Reserve from July 2012 to April 2013. Thirty monkeys (12 males,18 females)from a provisioned monkey group were observed via all-occurrence sampling. We found that the frequency of social play is significantly different among individuals in their first,second,or third years of life,and that play frequency negatively correlates with age. Choice of play partners and types of play also vary between age groups. Individuals are most likely to play with age-mates. The preferred games for individuals in the first year of life are chasing (67.5% ), while individuals in the second year of life most often catch (26.6% ),bite (25.3% ),and chase (40.5% )each other. We found no particular preference in the play of monkeys in their third year of life. On average,males spend more time playing than females. We interpret these patterns in the social play of R. bieti in light of the age/stage hypothesis,sex difference hypothesis,and locomotor development hypothesis.