Wild Helianthus annuus is native to North America but it naturalized in other parts of the world as well. Although the origin of exotic populations is uncertain, they have probably evolved very differently in different countries. To unravel the origin of invasive populations from Argentina and Spain, morpho logical and agro-ecological data of nine populations from central Argentina, six from Andalusia and one from Gerona were collected in their natural habitats during three exploration trips in 2007 and 2008. In Argentina wild H. annuus was found mainly in disturbed areas between roads and fences. In a few cases the populations were located on the margins of cultivated fields. The Argen tinean populations are spread across more than 50,000 m2 at a density of about 25 plants m-2. In Spain, the populations were found mainly in crop lands. The largest population covered about 1,500 m2 and comprised no more than 200 plants. The Argentinean populations had taller plants with a higher number of heads of small size, while the Spanish populations were character ized by bigger heads with wider ligules and bracts. Plants were shorter and leaf size was larger in Gerona than in Andalusia. Multivariate analysis differenti ated populations from Argentina and Spain by many traits. Wild-crop gene flow is likely the source of genetic variation among them. In Argentina, the popula tions keep the appearance of early wild introductions, while the Spanish popu lations are weedier and probably originated from pollen contamination of commercial seed with wild plants or crop-wild hybrids.