Geographically isolated from other C. albifrons taxa found east of the Andes, the Ecuadorian capuchin (Cebus albifrons aequatorialis) is a Critically Endangered primate that survives in a small number of localities in western Ecuador and extreme northern Peru. We assessed 11 forested areas in western Ecuador to determine presence/absence using a combination of on-foot searching and interviews with local informants. C. a. aequatorialis were present at seven of the sites surveyed, four of which represent new presence localities. We carried out extensive censuses of five small, private reserves to obtain estimates of population density and demographic information. We also examined home range characteristics and habitat selection at one well-studied site. Population densities based on absolute counts at these sites ranged from 2–22 individuals/km2 (median = 2.4). Jauneche, a 138 ha isolated fragment reserve with 22 individuals/km2, was a clear outlier. Although we observed some solitary individuals, C. a. aequatorialis live predominantly in multi-male multi-female social groups, with a mean group size of 13.9 (range 5–20). The composition of social groups was typical for Cebus: adult females outnumbered adult males slightly, and groups exhibited relatively high immature to adult female ratios (mean = 1.5). Home ranges were unusually large for the genus (507–561 ha). The capuchins exhibited strongest selection for mature forest near streams, although they also used degraded forest frequently. C. a. aequatorialis faces critical threats in the form of habitat loss, hunting, and harassment by farmers, but we suggest that some remaining populations have the potential to grow if effective protection can be established.