Sequences of dental development and eruption radiographically determined for 160 immature callitrichids were combined with nonontogenetic criteria in an investigation of callitrichid affinities. Marmosets (Callithrix and Cebuella) are distinct from tamarins (Leontopithecus and Saguinus) in both sequences of dental ontogeny and nonontogenetic characters. Callimico presents a tamarin-like pattern in its dental ontogeny and overall appearance. A new callitrichid classification which separates marmosets and tamarins into different subfamilies (Callitrichinae, Leontopithecinae and Callimiconinae) is proposed. Dental ontogenetic data suggest that callitrichids are derived platyrrhine taxa.