The pygmy marmoset population of a 3-km2 sample area of Amazonian lowland forest was censused and monitored intensively between September 1976 and January 1978. Floodplain forest constituted the habitat of Cebuella and supported a population density of 51.5 independently locomoting individuals (ILI) per square kilometer. The highest population concentration occurred along the edges of the river, where the density reached 274 ILI per km2. Adults comprised about one half of the total population. About 83% of the population lived in stable troops; the remaining 17% was made up by incipiently associated pairs and solitary individuals. Stable troops were made up of 1 breeding female, her mate, and her maturing offspring of up to four successive litters. Moreover, some troops contained 1–2 additional adult members. Troop size ranged from 2 to 9 ILI, with a modal size of 6 ILI. The births showed two annual peaks and the interbirth intervals ranged between 5 and 7 months. Infant survival was about 67%. Exudates (sap and gums) of trees and vines, insects and arachnids constituted the principal food resources of the population. The troops occupied exclusive home ranges of 0.2–0.4 ha. Several troops changed home range sites temporarily or permanently in the course of the study.