Seed size in tropical forests is expected to vary in relation to dispersal mode, recruitment requirements and the nature of seed enemies in each community. Some studies have emphasized an advantage of large-seeded species in environments subject to low rates of disturbance, but at the same time the number of dispersers tends to decrease as seed size increases. In this paper we describe how seed size affects the probabilities of seed dispersal by woolly monkeys at Tinigua National Park and we compare dispersal patterns among age/sex classes. The results are based on 1236 fecal samples recovered from focal individuals during two years. Woolly monkeys are able to swallow wide seeds; however, there seems to be a limit imposed by anatomical constraints that does not allow them to swallow seeds wider than 18 mm. Most of the seed mass dispersed by the monkeys falls in the categories between 6 and 12 mm wide. Seed selection in terms of mass was not observed for small-seeded species, but it was observed for some of the medium sized species (without any clear size preference) and there was a clear tendency to swallow relatively small seeds from the large-seeded species. Although we did not observe a difference in the number of seeds dispersed by deposition among age/sex classes, juveniles disperse a lower seed mass than adult animals. These differences highlight that different age/sex categories play distinct ecological roles in terms of seed dispersal processes. Seed selection by primates might impose selection pressures in seed size traits, but more studies are necessary to ascertain their potential evolutionary role.