Genetic variation in the human serotonin system has long been studied because of its functional consequences and links to various neuropsychiatric and behavior-related disorders. Among non-human primates, the common marmosets (Callithrix jacchus) and tufted capuchins monkeys (Cebus apella) are becoming increasingly used as models to study the effects of genes, environments, and their interaction on physiology and complex behavior. In order to investigate the independent functions of and potential interactions between serotonin-related genes, anxiety and neuropsychiatric disorders, we analyzed the presence and variability of the serotonin transporter gene-linked polymorphic region (5-HTTLPR) in marmoset and capuchin monkeys. By PCR and using heterologous primers from the human sequence, we amplified and then sequenced the corresponding 5-HTT region in marmosets and capuchins. The resulting data revealed the presence of a tandem repeat sequence similar to that described in humans, but unlike humans and other Old World primates, no variable length alleles were detected in these New World monkeys, suggesting that if serotonin transporter is involved in modulating behavior in these animals it does so through different molecular mechanisms. Am. J. Primatol. 74:1028-1034, 2012. © 2012 Wiley Periodicals, Inc.