Nucleolar organizer regions were analyzed in four different species of Alouatta (Atelidae, Platyrrhini) using silver staining. They showed a high variability, appearing in dif-ferent numbers and positions, with a singular variation not found in other primate genera so far. Hence, the number of NOR-sites varied from four to six, while the number of NOR-bearing chromosomes were two, four or six. The comparison of these data with previous ones suggests that A. fusca clamitans and A. caraya, with two small pairs of acrocentric chro-mosomes with a NOR-site in the proximal region of the long arm, have a conservative trend from a hypothetical ancestor, while A. behebul belzebul and A. seniculus macconelli have derived states. Moreover, it seems that the processes of rDNA amplification sites occurred in-dependently at least four times during the irradiation of this genus. These processes could be due to transposition events or to the amplification of inactivated NORs containing a relatively small number of rDNA copies. Our results reinforce the chromosomal phylogeny proposed to this genus, corroborating the basal position of A. caraya, and the position of A. belzebul nigerrima, closer to A. seniculus group.