Black ear-tufted marmosets are diurnal, sleeping at night in tree holes or other shelters. They live in family groups of 4-15 individuals, having overlapping home ranges and do not defend territories. Dominance in the group is maintained through aggressive interaction. Marmosets are usually docile and gentle in captivity. Sudden noises or movements sometimes cause them to panic.
Marmosets run quadrupedally along horizontal branches and leaping between thin terminal supports.
They have acute sight, good hearing, and good smell. Facial expressions are indicated mainly by lip movements. Marmosets are fastidious in the care of their pelage and engage in both individual and mutual grooming, the former being more common. Many tree-living animals have a group of long bristles on each wrist. These are tactile hairs, and marmosets are the only members of the higher primates to have them.