The phylogenetic affinities of the neotropical Goeldi's monkey (Callimico goeldii) have long been a matter of debate. Whereas most morphological evidence appears to place Callimico in a sister group position relative to the Callitrichidae, genetic studies place C. goeldii within the Callitrichidae and suggest that it is more closely related to marmosets than to tamarins. The present study presents the first radiographic analysis comparing the secondary limb bone ossification of newborn C. goeldii with representatives of the marmosets (Callithrix jacchus) and tamarins (Saguinus oedipus). The state of secondary ossification of the epiphysis and short bones is classified into three different ontogenetic stages. Our results reveal that in terms of the number of ossification centers, C. goeldii is significantly closer to C. jacchus than to S. oedipus. This is the first morphological study to support the findings of molecular studies, and the results suggest that C. goeldii is more closely related to marmosets than to tamarins. Am. J. Primatol. 69:1–14, 2007. © 2006 Wiley-Liss, Inc.