The effect of forest fragmentation on ranging and diet was studied in four groups of the endangered hoolock gibbon in four forest fragments and compared to two control groups in a large forest in Assam, NE India. A survey of hoolock gibbon populations was also carried out at 14 sites where encounter rate, group size and age-sex ratios were compared between forest size classes. Habitat variables were influenced more by disturbance level than fragment size. Disturbed fragments had lower canopy continuity, food tree diversity, and important food sources like lianas and climbers. Home range size was not correlated to the size of the forest. Gibbons in the two smallest fragments had almost no fruit in their diet at the end of the dry season. Gibbons in the disturbed fragments spent the least time in social interactions and called less frequently during times of fruit shortage indicating that they make trade-offs between conserving energy and investment into group bonding or territorial defense. Encounter rates of gibbon groups were lower, and group sizes smaller, in the fragments than in the large forest.