The common marmoset (Callithrix jacchus) is unique among the primates in its small body size, reproductive efficacy, and characteristic social behavior, making it useful as an animal model in neuroscientific research. To assess the brain serotonergic systems, we investigated the binding of [(11)C]-3-amino-4-(2-dimetylaminomethyl-phenylsulfanyl)-benzonitrile ([(11)C]DASB) to brain serotonin transporter (SERT) in conscious common marmosets using positron emission tomography (PET), and compared with findings for rhesus monkeys. Both species showed globally similar distribution patterns of [(11)C]DASB uptake in the brain, with highest activity in the midline of the brain and lowest in the cerebellum, and higher activity in some subcortical regions than in surrounding cortex, while the common marmoset brain showed almost equal or rather higher binding potential (BP) values (BP(ND)) in cortical regions and hippocampus, and lower BP(ND) than the rhesus monkey brain in some subcortical regions. Test-retest reproducibility of BP(ND) at an interval of several months was high, indicating reliable and stable measurements of serotonin transporters in both species. These results suggest that SERT imaging by PET with [(11)C]DASB under conscious state is valuable for investigating the physiological serotonergic functions in common marmosets (182).