The karyology was studied in 9 species of cetaceans, viz. in 5 odontocetes and 4 mysticetes. Chromosome measurements and idiograms of 8 species are presented. Comparisons were made between the karyotypes of the different species on the basis of conventional staining methods as well as by autoradiography and by Q-, G-, and C-banding techniques. All cetaceans so far studied have 2n = 44, except the sperm and pygmy sperm whales, which have 2n = 42 and karyotypes entirely different from those of the 2n = 44 species. All of the latter, except the killer whale, have closely concordant karyotypic morphology. Specific attention was payed to the C-heterochromatin, both because of its large amount and its striking pattern. The amount of C-heterochromatin varied from 10–15% of the total chromosome length in the odontocetes to 25–30% in the mysticetes. In both of them the distribution of C-heterochromatin in the karyotypes was mainly interstitial and terminal and to a lesser extent centromeric. Size heteromorphism in C-bands was frequently observed between the homologues of the same pair. The pronounced karyotypic agreement between the odontocetes and mysticetes, both in general chromosome morphology and in C-band pattern, is incompatible with the theory of a diphyletic origin of the Odontoceti and Mysticeti.