The golden-headed or Cat Ba langur (Trachypithecus poliocephalus poliocephalus) is endemic to Cat Ba Island in northern Vietnam. Two free-ranging groups were studied for 22 days of data collection. Behavioral data were collected via instantaneous scan sampling for determining the time budget, activity pattern and vertical ranging pattern of these groups. The langurs spent two thirds of their time resting, 15% foraging, 11% moving and 8% in social activities. Time budgets differed between age classes, with infants and juveniles spending more time being social. The activity pattern of the groups revealed that the langurs rested mostly during the morning and at midday when most of the social behavior was also observed. Foraging and moving occurred more often in the morning and afternoon, with foraging being more pronounced later in the day. With respect to their ranging, the langurs spent most of their time in the higher elevations, but foraged more in the lower elevations of the hills where they live. In this study, we show that the best time to census this Critically Endangered species is from sunrise till 12:00 am.