We evaluate the impact of very small and isolated forest fragments on the common marmosets home range, diet, and activity patterns, in the northeastern Atlantic Forest of Brazil. Three groups were studied in three forest fragments, from January to October 2010, totaling 360 hours of observations and 1,080 field-hours. Systematic observations were recorded using Instantaneous Scan Sampling, and a checklist of the items exploited was built through ad libitum observations. We recorded location of the groups and calculated home range. We recorded 11,639 scans and 236 ad libitum feeding records. 83.4% (n=10) of food items were plant species, the only animal protein was from insects (n=2; 16.6%), and the diet was based almost exclusively on gums. Mean home range was 5.5 ha, mean daily path length was 1,167 meters, and no differences were detected between seasons. Resting dominated their activity budget and varied between seasons. Common marmosets survived in this environment through a remarkable increase in their exploitation of tree gums (up to 98% of their feeding bouts) to compensate for the lack of food, in home ranges slightly larger than in the literature. Thus, they travelled and foraged less than expected and rested more since food was easily obtained.