The chromosome numbers of 209 individuals of Aster cordifolius L. from ca. 130 localities from the provinces of Quebec and New Brunswick were determined; 122 were diploid and 87, tetraploid. Four populations are mixed, but no triploid was found; experimental crosses beween the two cytotypes failed. Genic exchanges do not appear to exist between the two races, at least not via normal gametes. In Quebec, the two cytotypes are sympatric in the Richelieu valley; west of this area, only tetraploids are found, and east of it, only diploids. This distribution pattern continues the one observed in Ontario, New York State, and New England, where the contact zone is, however, wider. No obvious ecological differences were noted between the two chromosomal races. Postglacial migration on either side of the Appalachian–Adirondack axis, tetraploids to the west and diploids to the east, appears to explain the distribution of cytotypes of A. cordifolius in Quebec.