Ecological factors are known to influence the activity budgets of Yunnan snub-nosed monkeys (Rhinopithecus bieti). However, little is known about how activity budgets vary between age/sex classes, because the species is difficult to observe in the wild. This study provides the first detailed activity budgets subdivided by age/sex classes based on observations of the largest habituated group at Xiangguqing in Baimaxueshan Nature Reserve. This study was conducted from June 2008 to May 2009. We found that adult females spent more time feeding (44.8%) than adult males (39.5%), juveniles (39.1%), and infants (14.2%). Adult males allocated more time to miscellaneous activities (12.5%) than did adult females (3.8%). Infants were being groomed 6.9% of the time, which was the highest proportion among all age/sex classes. Adults spent more time feeding, while immature individuals allocated more time to moving and other activities. There are several reasons activity budgets may vary by age/sex class: 1) differential reproductive investment between males and females; 2) developmental differences among the age categories; 3) social relationships between members of different age/sex classes, particularly dominance. In addition, group size and adult sex ratio may also impact activity budgets. These variations in activity budgets among the different age/sex classes may become a selective pressure that shapes the development and growth pattern in this species.