The chromosome numbers of plants of theAchillea millefoliumcomplex from 27 locations in North America are reported. The levels of polyploidy found are correlated with diameters of the pollen grains. Measurements of pollen grains from more than 300 herbarium specimens are used as an indication of the occurrence of tetraploid and hexaploid species ofAchilleain Canada. The results of some crossing experiments are also reported. It is concluded that the commonAchilleaof North America is tetraploid and is the nativeA. lanulosaNutt. It extends from the Pacific to the Atlantic coasts, as far south as California in the west and Pennsylvania in the east, and to northern Canada and Alaska. Another native species, hexaploidAchillea borealisBong., extends along the Pacific coast from southern California to the Aleutian Islands and along the northern shorelines of Alaska and Canada. A Eurasiatic hexaploid,A. millefoliumL. s. str., has been sparingly introduced into Newfoundland, Prince Edward Island, Nova Scotia, and perhaps elsewhere along the Atlantic seaboard. In eastern Canada a male-sterile purple-flowered hexaploid also occurs. This hexaploid escapes from cultivation and forms localized patches. It undoubtedly has been selected from the EurasiaticA. millefoliumL. s str.