The morphology and specific differences ofCirsium undulatum(Nutt.) Spreng. and ofC. flodmanii(Rydb.) Arthur are described and their Canadian distributions are reported in detail. The chromosome numbers areC. undulatumf.undulatumand f.albumFarwell, 2n = 26;C. flodmaniif.flodmaniiand f.albiflorumD. Löve, 2n = 22. The origin of four North American species ofCirsiumthat do not follow the world-wide base number 17 is discussed; it is postulated that reduction in number has occurred by translocations. The chromosomes of species with reduced numbers are larger than those of the unreduced species but the total length of the chromosomes of both groups is approximately the same.