A variation in the karyotype ofAlouatta fusca clamitamfrom different geographical ranges is presented. Blood samples of six captive specimens from different States of Brazil were analyzed. Distinct diploid numbers were found: 2n=52, 2n=45 and 46. The odd number was due to Y-autosome translocation. The comparison of the G-banding patterns with previous published reports of 2n=49 and 50 allowed us to identify Robertsonian rearrangements and pericentric inversions as the most important chromosomal rearrangements involved in the differentiation of the observed cytotypes. The finding of specific cytotypes associated to different geographical ranges suggests a northern-southern division ofAlouatta fusca damitans,due to a possible reproductive isolation between different chromosomal complements.The fixation of such rearrangements could have occurred during the existence of the quaternary refuges, when the populations have been disrupted in smaller denies.