The effect of nest box availability on the circadian activity rhythm (CAR) was compared in a diurnal primate kept in two conditions: with and without nest box, in a light-dark (LD) cycle (12:12) and constant light (LL). Marmosets kept in LD without nest box showed a longer duration of the activity phase (alpha), smaller day-to-day variability in activity onset and offset and higher levels of activity, particularly towards the end of the activity time. In LL, all animals maintained with nest box expressed free-running CAR, but without nest box, 3 of the 4 individuals showed relative coordination with the outdoor LD cycle, possibly due to social synchronization with captive marmosets maintained outdoors. All animals showed longer alpha and higher levels of activity without nest box. Thus, the availability of a nest box is an important aspect in maintaining marmosets, influencing CAR expression in this diurnal primate in LD and LL conditions, possibly due to differences in light exposure that induces masking. Further studies are needed to evaluate the effects of the unavailability of a nest box on CAR in LL conditions.