CT imaging was undertaken on the skull of approximately 20-Myr-old Miocene Tremacebus harringtoni. Here we report our observations on the relative size of the olfactory fossa and its implications for the behavior of Tremacebus. The endocranial surface of Tremacebus is incomplete, making precise estimate of brain size and olfactory fossa size imprecise. However, olfactory fossa breadth and maximum endocranial breadth measured from CT images of one catarrhine species and eight platyrrhine species for which volumes of the olfactory bulb and brain are known show that the osteological proxies give a reasonably accurate indication of relative olfactory bulb size. Nocturnal Aotus has the largest relative olfactory fossa breadth and the largest olfactory bulb volume compared to brain volume among extant anthropoids. Tremacebus had a much smaller olfactory fossa breadth and, by inference, bulb volume--within the range of our sample of diurnal anthropoids. Variations in the relative size of the olfactory bulbs in platyrrhines appear to relate to the importance of olfaction in daily behaviors. Aotus has the largest olfactory bulbs among platyrrhines and relies more on olfactory cues when foraging than Cebus, Callicebus, or Saguinus. As in other examples of nocturnal versus diurnal primates, nocturnality may have been the environmental factor that selected for this difference in Aotus, although communication and other behaviors are also likely to select for olfactory variation in diurnal anthropoids. Considering the olfactory fossa size of Tremacebus, olfactory ability of this Miocene monkey was probably not as sensitive as in Aotus and counts against the hypothesis that Tremacebus was nocturnal. This finding accords well with previous observations that the orbits of Tremacebus are not as large as nocturnal Aotus.