Because of their small body size, tamarins are potentially vulnerable to a wide range of predator species. This paper examines the general patterns of predator avoidance of the Avila-Pires saddle-back tamarin (Saguinus fuscicollis avilapiresi) and the red-cap moustached tamarin (Saguinus mystax pileatus) in the upper Urucu river, Amazonas, Brazil. One highly stable mixed-species group of these species suffered an average of 1 attack by carnivorous raptors every 8.8 days of activity. Although none of these attacks was successful tamarins occasionally fall prey to raptors or terrestrial predators. The two tamarin species exhibited a high degree of coordination in their anti-predation tactics. Saddle-back tamarins accounted for most of the vigilance and detection of terrestrial and scansorial threats, whereas moustached tamarins scanned for and detected most aerial and arboreal threats. Of the two species, however, moustached tamarins contributed a significantly greater proportion of scanning time after predation attempts, during pre-retirement periods and at other times. Moreover, their substantially larger groups (1.6 times larger than those of saddle-back tamarins) accounted for a further increase in their effective vigilance effort. The anti-predation benefits gained by saddle-back tamarins from joining mixed-species groups with moustached tamarins thus appeared to be greater than the reverse, because the latter provide greater protection to the group, particularly against aerial sources of threat. However, the greater vigilance of saddle-back tamarins towards potential terrestrial predators cannot be discounted, because these may on rarer occasions pose a real threat to either species.(ABSTRACT TRUNCATED AT 250 WORDS)