Karyotypes of lake whitefish, Coregonus dupeaformis, and various Great Lakes cisco species, lake herring or cisco, C. artedi, bloater, C. hoyi, blackfin cisco, C. nigripinnis, and shortjaw cisco, C. zenithicus, were analyzed using Giemsa staining, C-banding, silver staining, and Chromomycin A3 fluorescence. Coregonus dupeaformis had a karyotype (2n = 80, NF = 98) indistinguishable from that described for C. laveretus from Europe, with one pair of nuclear organizer regions (NORs) on the short arms of a small acrocentric chromosome pair and a second NOR on the short arms of another acrocentric chromosome pair. This species displayed a size polymorphism involving the heterochromatic short arm of the largest metacentric-submetacentric pair. All four cisco species had similar karyotypes (2n = 80, NF = 98), with the exception of some C. artedi from Lake Superior, which were heterozygous for a chromosome fusion involving the NOR chromosomes (2n = 79). One NOR was located pericentromerically on the long arm of the second largest acrocentric chromosome and a second NOR on the short arms of another acrocentric chromosome pair in several individuals. C-banding revealed that the difference in chromosome morphology between lake whitefish and the ciscoes probably resulted from amplification of heterochromatin on the short arms of one pair of large submetacentric-metacentric chromosomes.