Clasen, B. M., Moss, N. G., Chandler, M. A. and Smith, A. G. 2011. A preliminary genetic structure study of the non-native weed, common tansy (Tanacetum vulgare). Can. J. Plant Sci. 91: 717-723. Common tansy is an herbaceous perennial member of the Asteraceae and is considered a weed in North America. Common tansy was introduced deliberately for use as a funerary herb, medicine, preservative, and animal and insect repellent. It is known to escape cultivation and invade disturbed areas, spreading both sexually and asexually. This paper reports a preliminary analysis of the genetic structure of 10 invasive common tansy populations in Minnesota and Montana, USA, and Alberta, Canada. Ninety polymorphic loci were found using six inter simple sequence repeat (ISSR) primers used to amplify DNA from 40 individuals from 10 discrete populations. The diversity within and among populations was assessed using the Dice coefficient of similarity and AMOVA. The AMOVA showed that diversity within populations was generally high and that there was relatively small variation among populations. An unweighted pair-group with arithmetic mean (UPGMA) dendrogram was constructed based on the distance between populations, and demonstrated substantial and distinct clustering of a population from Ramsey County, Minnesota. A principal coordinates analysis clustered all individuals from Ramsey County distinctly from other individuals, indicating a possible limited gene flow among this population and the other populations sampled in this study. Understanding genetic diversity and the distribution of diversity within and among populations may help predict the potential for successful management of common tansy populations in North America.