The chromosome constitution of Alouatta belzebul was studied with G-, C-, and silver staining. In ten specimens identified as Alouatta belzebul, the chromosome constitution of males (2n = 49) differed from that of females (2n = 50) owing to a Y-autosome translocation. In another single female specimen, probably Alouatta belzebul nigerrima, the diploid chromosome number was also 50, though its karyotype was drastically different from that of Alouatta belzebul belzebul. Chromosome studies, taken together with phenotypic and field observations, suggest that Alouatta belzebul belzebul is phenotypically variable in respect to pelage coloration. This attribute is therefore unreliable for the precise identification of Alouatta belzebul subspecies. Conversely, relatively minor phenotypic differences, allowing for the characterization of subspecies within a same species, coexist with unparalleled, drastic karyotypic divergence. These findings clearly question gross morphological attributes as discriminative characteristics of Alouatta belzebul subspecies.