Os cari?tipos referentes a quatro machos de Alouatta fusca clamitans oriundos do Rio de Janeiro foram analisados atrav?s de t?cnicas de bandamento G, C e NOR. O n?mero dipl?ide em todos os esp?cimes foi igual a 49, com a presen?a de tr?s cromossomos n?o pareados. A compara??o dos padr?es de bandamento G com esp?cimes previamente descritos com 2n = 50 revelou a ocorr?ncia de uma transloca??o do tipo Y-autossomo, modificando o sistema cromoss?mico de determina??o sexual para o tipo m?ltiplo, X1X2Y/X1X1 X2X2. Os blocos de heterocromatina constitutiva se distribu?ram na regi?o pericentrom?rica de todos os cromossomos; segmentos intercalares e telom?ricos foram visualizados em um par acroc?ntrico e em outro submetac?ntrico, respectivamente. As regi?es organizadoras de nucl?olo se localizaram no bra?o longo de dois pares de pequenos acroc?ntricos. ; ABSTRACT: The chromosome complements of four males of Alouatta fusca clamitans, caught in Rio de Janeiro State, Brazil, were analyzed by G-, C-, and NOR-banding techniques. The diploid number found was 49 in all the specimens. The presence of a heteromorphic pair of submetacentric chromosomes in the analyzed specimens, not present in males and females with 2n = 50 previously reported, and its G-banding pattern, led us to assume that this pair is involved in a Y-autosome translocation. Thus, the sex determination system appears modified to X1X1X2X2 /X1X2Y. Heterochromatic segments were found in the pericentromeric region of all the chromosomes, in the telomeric region of the short arm in pair 2, in the complete length of the short arm of pairs 5 and 6 and in the intercalary region of the long arm in pair 17. The nucleolar organizer regions were situated in the intercalary region of the long arm in two small acrocentric pairs.