The Endangered red-shanked douc Pygathrix nemaeus is a charismatic Asian colobine endemic to Vietnam, Cambodia and Lao People's Democratic Republic (PDR). The largest population is found in central-eastern Lao PDR but it has never been quantified. We conducted line-transect surveys in Nakai–Nam Theun National Protected Area in 2011–2012. We used distance sampling combined with a habitat suitability model to estimate group density and group abundance in the area. Our analysis yielded a density of 2.8 (range 1.9–4.1) groups per km2 and c. 4,420 groups within the predicted c. 1,600 km2 of suitable habitat in Nakai–Nam Theun. These are the only available data for P. nemaeus in Lao PDR to date. We discuss the limitations of our survey, which involved potential violations of the distance sampling method, and highlight the challenges of using this method for primates. We also consider the importance of assessing limitations in distance sampling studies, to evaluate reliability and facilitate comparisons. The population of P. nemaeus in Lao PDR is under threat from illegal hunting and is not the focus of any specific conservation action. To avoid a repetition of the decline the species has already experienced in Vietnam, and to secure the species' global survival, conservation projects need to target the population in Nakai–Nam Theun. A long-term conservation and research project in this key biodiversity area will also benefit other threatened and unique co-existing species.

More than 70 h of positional behaviour data were collected on the red-shanked douc langur Pygathrix nemaeus, Delacour's langur Trachypithecus delacouri, Hatinh langur Trachypithecus laotum, and the white-cheeked crested gibbon Hylobates leucogenys in January and February of 2001 at the Endangered Primate Rescue Center of Cuc Phuong National Park, Ninh Binh Province, Vietnam. Equal amounts of instantaneous and continuous bout data were collected for each of the four species. All animals were housed in enclosures of 10×5×3.5 m with similar substrates, and lived in small, species-appropriate social groups. The positional behaviour of white-cheeked crested gibbons was as expected with >80% of locomotor bouts and 60% of postures being suspensory in nature. The positional behaviour of the Delacour's and Hatinh langurs was similar to that reported in the literature for other Asian colobines with >90% of the locomotor bouts being quadrupedal walking, running, and bounding on tops of arboreal supports. Postures were dominated by sitting in and sitting out with forelimb suspension associated with <10% of these behaviours. The positional behaviour of the red-shanked douc langur is intermediate in nature between the Delacour's and Hatinh langur and the white-cheeked crested gibbon. Locomotion is nearly evenly divided between arm-swinging (46%) and quadrupedal walking and running bouts (54%). Forelimb suspensory postures were three times more frequent for the red-shanked douc langur than they were for the Delacour's langur.