The role of polyploidy in facilitating plant invasions is not fully understood.  In this study we examined the effects of polyploidy on native and invasive populations of Solidago gigantea.  S. gigantea is native to North America and introduced to Europe.  Three cytotypes are found in the native range (diploid, tetraploid, and hexaploid).  All 3 cytotypes have been reported in Europe, although their abundance and distribution have not been well-characterized.  We determined the chromosome number of  80 US and 80 European genotypes drawn from 10 populations in the US and 20 populations in Europe.  Chromosome numbers were determined using root tip staining and slide press squashes.  Plants were grown in common gardens for several different experiments carried out over several years.  Plants were propagated from year-to-year using rhizome cuttings, so the same genotypes were used in successive experiments.  We found all 3 cytotypes in our North Amercan samples, but only diploids and tetraploids in the European populations.  We tested for the combined effects of ploidy and continental source on resistance to insects and leaf pathogens, secondary chemistry, physiological and morphological characters (photosynthesis, conductance, leaf area and specific leaf area), and plant fitness ( rhizome and infructescence biomass).  Some plant traits were strongly affected by ploidy but not by continental origin, while other traits were influenced by origin but not ploidy. Other traits were jointly affected by both continental origin and ploidy.  These results can help provide insights into evolutionary changes occurring in S. gigantea in its invasve range in Europe.