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Two-photon imaging with genetically encoded calcium indicators (GECIs) enables long-term observation of neuronal activity invivo. However, there are very few studies of GECIs in primates. Here, we report a method for long-term imaging of a GECI, GCaMP6f, expressed from adeno-associated virus vectors in cortical neurons of the adult common marmoset (Callithrix jacchus), a small New World primate. We used a tetracycline-inducible expression system to robustly amplify neuronal GCaMP6f expression and up- and downregulate it for more than 100days. We succeeded in monitoring spontaneous activity not only from hundreds of neurons three-dimensionally distributed in layers 2 and 3 but also from single dendrites and axons inlayer 1. Furthermore, we detected selective activities from somata, dendrites, and axons in the somatosensory cortex responding to specific tactile stimuli. Our results provide a way to investigate the organization and plasticity of cortical microcircuits at subcellular resolution in non-human primates. Long-term two-photon calcium imaging has been challenging in non-human primates. Sadakane etal. use an inducible expression system to visualize cortical neurons in adult marmosets. They show that the same neuronal population can be followed over 100days and neuronal responses to tactile stimulation can be imaged at subcellular resolution.