Two families of tandemly repeated satellite DNAs were isolated from the neotropical primate Callimico goeldii (Goeldi's marmoset). One satellite, CgoA, is over 70% A + T and has a monomer length of 338 bp. The other satellite, CgoB, is 50% A + T and has a monomer length of 916 bp. Both CgoA and CgoB hybridize strongly with Callimico DNA, but not with the DNA of other new and old world primates. Based upon a neutral substitution rate of 1.5 X 10(-9)/site per year for primates, sequence data from 15 CgoA monomers indicate that the tandem array is at least 30 million years old. Since no other neotropical primate has amplified CgoA sequences, the data suggest that the ancestor of Callimico separated from the other neotropical primates at least 30 million years ago. This value is about fourfold larger than the value of 7-9 million years derived from immunological data by Sarich and Cronin (1980). Possible reasons for this discrepancy are discussed.