Callitrichines share several morphological features that appear to be derived among anthropoid primates. One view maintains that some of them are the consequence of a rapid reduction in body size in the common ancestor of callitrichines. This hypothesis predicts that callitrichines should have relatively large teeth for their body size in comparison to other platyrrhines. Dental metric data from 18 platyrrhine species, including 4 callitrichines, is used to test this hypothesis. Callitrichine tooth size is compared both to empirical regressions of tooth size against body weight for noncallitrichine platyrrhines and to a prediction of geometric similarity. In neither comparison do callitrichines as a group show significantly greater tooth size than other platyrrhines. In fact, three of the four genera seem to have relatively small teeth for their body size. While this study fails to support the hypothesis that the common ancestor of callitrichines underwent a rapid reduction in body size, it neither proves nor disproves the hypothesis that they are smaller than their last common ancestor.