Uakari monkeys from the Amazon River basin are the most specialized primates in the Neotropical region. Their short tails, peculiar dentition and external morphology are strikingly divergent from most other platyrrhines. Furthermore, their geographical range is small and restricted to the forests in the floodplains of Amazonian Rivers. It is the only frugivorous primate able to live within the young $varzeas$ (white-water river floodplain) of the Amazon. A 20-month study of the ecology of the white uakaris $(Cacajao\; calvus\; calvus)$ was carried out in the floodplain located between the Japur$a´$ and Amazon Rivers in Brazilian Amazonia. The vegetation is unique in the way that it is able to cope with a 12-metre annual change in water level. This annual variation in water level is probably the most important overall ecological mechanism in $varzea$, influencing the temporal and spatial patterns of distribution of fruits. Several aspects of the ecology and ranging behaviour of the uakaris are associated with the temporal and spatial patterns of distribution of potential food sources: the seeds of immature fruits. Subdividing fruits into separate morphological classes is of fundamental importance for understanding this variation, which is largely influenced by the changes in water level. Foods of high quality, high energetic value and low in secondary compounds, are extremely important at most times of the year due to the Uakari's gut structure, metabolic requirements, use of space and, especially the topography of their habitat. Comparisons are made with another pithecine of similar morphology and diet, but which lives in the dry terra firrne forests. Based on their present distribution and ecological preferences, an attempt is made to trace the evolutionary history of these two largest pithecines. Furthermore, the role of riverine habitats in the zoogeographic and speciation patterns of primates within Amazonia is discussed.