The aim of this study was to understand plasticity and resilience in the behavioral ecology of one of India's most endangered primates, the golden langur (Trachypithecus geei) inhabiting forest reserves in Assam. Our survey revealed that golden langur populations in Assam are isolated; restricted to just pockets of suitable habitat. Their forests face severe human pressure mainly due to illegal logging, hunting, and widespread invasion for both commercial interests and by displaced refugees. An extensive survey using a stratified random walk transect method estimated a total population of 943 individuals living in 96 groups. About 40% of the langurs recorded were immature and the average group size was 9.8. Adult sex ratio was 1.5 to 5.0 adult female for each adult male. Four groups inhabiting forests with varying degree of disturbance were monitored using focal animal sampling for 1,728 hours during 12 months. Records on activity budgets, food and feeding behavior, and reproductive and ecological profiles indicated significant differences between them, attributed to differences in habitat quality. Small groups, isolation, proportionately few infants and juveniles, and habitats of increasingly poor quality are all parameters suggesting further decline in the golden langur population of the region. Conservation initiatives focusing on research, education, and community participation are show considerable success.