The titi monkeys (Callicebus spp.) are a large, diverse genus of platyrrhines, widely distributed in tropical South America. The genetic variability of these monkeys is still relatively poorly known, especially at the population level. In the present study, four heterologous microsatellite markers were used to investigate genetic diversity in 23 individuals from a wild population of red-bellied titis (Callicebus moloch) in eastern Amazonia. An unexpectedly low level of diversity was found. The average number of alleles was 8.75 (range: 5-15), and the average heterozygosity was 0.33 (range: 0.09-0.65). This preliminary information suggests a reduction of the potential for long-term survival of the population and indicates the putative necessity of implementation of a species conservation program.