This paper presents Q- G- R- and C-bands karyotypes ofred howler from Colombia, based on analyses of karyotypes oftwelve caught specimens besides a female and a male Q banding karyotypes (Yunis et al. 1976) reevaluated. We reappraisal X chromosome morphology and sex determination chromosome mechanism of Alouatta seniculus seniculus. This work shows X chromosome is a nearly metacentric medium sized chromosome (4.85 %TCL), similar to other primates X chromosome from Cebus, Aotus and Ateles genus up to now studied, as well as human X chromosome. Further in males, Y chromosome undergone a translocation onto autosome 3, named X1X2Y1Y2• Al! specimens studied shows 44 chromosomes diploid number.