The white-fronted spider monkey, Ateles marginatus, is endemic to Brazilian Amazon between the Tapajós and Xingu Rivers, and is classified as endangered due primarily to habitat loss. This study presents data on the distribution and relative abundance of the species in the northwestern corner of its distribution, on the lower Tapajós River, centered on the 545,000 ha Tapajós National Forest (Flona Tapajós). Quantitative data were collected in line transect surveys at 14 sites, and confirmatory interviews were conducted at these, and a further four sites. The results indicate that A. marginatus is extinct or extremely rare at sites in the northern extreme of the study area, but is relatively widespread further south, except in very small fragments (<100 ha). While the present-day occurrence of A. marginatus is clearly affected by habitat fragmentation, the species' original distribution appears to have been characterized by local lacunas, determined by factors relating to habitat quality. While hunting pressure does not seem to be a major threat at the present time, competition with a second atelid, Alouatta discolor, may have reinforced the effects of habitat disturbance in some areas, especially smaller fragments. Grouping patterns were typical of the genus, including an apparent female bias in the sex ratio of adults. Overall, the results indicate an ongoing decline in the occurrence of A. marginatus within the study area, local extinction, especially in smaller fragments, and an urgent need for the establishment of effective conservation measures.