The karyotype of two species of birds of prey, Spizaetus tyrannus (2n=68) and Coragyps atratus (2n=80) are described for the first time. Moreover, a reanalysis of three other species is presented. Chromosomal data were compared to phylogenetic arrangements proposed for Fal- coniformes. Cytotaxonomic studies pointed towards a great difference between Cathartidae, which retained a standard avian karyotype, and species of the families Accipitridae and Falconidae, which showed derived karyotypes. Cytogenetic analysis also revealed a dichotomy between Falconidae and Accipitridae. K eywords : Cathartidae, Falconidae, Accipitridae, cytotaxonomy, Coragyps atratus, Spizaetus tyrannus.