Wild common marmosets (Callithrix jacchus) feed on fruits, insects, and gums, all of which provide different digestive challenges. Much of the ingested mass of fruits consists of seeds. In general, seeds represent indigestible bulk to marmosets and could inhibit feeding if they are not eliminated rapidly. In contrast, gums are (Winked polysaccharides that require microbial fermentation. Their digestion would benefit from an extended residence time within the gut. Earlier research found that mean retention time (MRT) for a liquid digestive marker (cobalt EDTA) was significantly longer than MRT for a particulate marker (chromium-mordanted fiber), suggesting that common marmosets preferentially retain liquid digesta. We conducted two four-day-long digestion trials on 13 individually housed adult common marmosets fed a single-item, purified diet in order to examine the relations among MRT of cobalt EDTA and chromium-mordanted fiber, food dry matter intake (DMI), and apparent digestibility of dry matter (ADDM). We compared the MRT values with the data from the previous study mentioned above and a study using polystyrene beads. There were no significant correlations among MRT, ADDM, or DMI, although increases in DMI between trials were associated with decreases in MRT. ADDM was consistent within individuals between trials; but the mean values ranged from 75.0 to 83.4% among individuals. We found no difference in MRT between the liquid (17.5 +1.6 hr) and particulate (17.9+1.4 hr) markers. Although these values were not significantly different than found previously, the MRT for chromium-mordanted fiber tended to be longer. This probably reflects the relatively small size of the chromium-mordanted fiber particles used in this study. An inverse relationship between particle size and MRT was evident; the mean MRT of polysterene beads, the largest marker, was only 8.3 +1.5 hr. Marmosets appear to retain liquids and small particles within the gut longer than large particles. Am. J. Primatol. 71:957-963, 2009.