Introduced species inevitably experience novel selection pressures in their new environments as a result of changes in mutualist and antagonist relationships. While most previous work has examined how escape from specialist enemies has influenced herbivore or pathogen resistance of exotic species, post-introduction shifts in exotic dependence on mutualists have not been considered. In a common environment, we compared dependence on AM fungi of North American and European populations of Hypericum perforatum (St. John's Wort), a forb native to Europe. Introduced North American populations responded less to inoculation with AM fungi than did European populations. Root architecture was strongly correlated with mycorrhizal response, and introduced populations had finer root architecture than native populations. Finally, introduced populations exhibited decreased root and increased reproductive allocation relative to European populations, consistent with a transition to a weedier life history; however, biomass allocation patterns were uncorrelated with mycorrhizal response. These findings are the first demonstration of a genetically based reduction of mycorrhizal dependence and shift in root architecture in an introduced species.