Morphological descriptions, chromosome numbers, and Canadian distribution ofCirsium muticumandC. discolorare given. The related speciesC. altissimumof the United States is similarly considered. The chromosome numbers of the species are:C. muticum,2n = 20;C. discolor, 2n = 20;C. altissimum, 2n = 18. Extra chromosomes, possibly to be considered as accessories, were found inC. muticumandC. discolor. A Florida collection ofC. muticumhad the apparently triploid number 2n = 30. The morphological and cytological evolution of the group is discussed.