The karyotypes of six killer whales were studied by banding techniques. A striking accumulation of C-heterochromatin has occurred in the Oricinus karyotype. The four telocentric pairs characteristic of the 2n = 44 cetacean karyotypes have become masked due to the accumulation of heterochromatin in the short arms. Conspicuous C-band polymorphism occurred in the materials studied, rendering each specimen karyotypically unique. Owing to the amount and variation of C-heterochromatin in the Orcinus karyotype, access to C-banded karyotypes was essential for the evaluation of the G-band pattern. The G-band pattern of Orcinus was found to be closely similar to that of Stenella. The results indicate an evolution of the Orcinus karyotype from the karyotypes characterizing other delphinids.