29Coastal sand dunes are dynamic ecosystems with elevated levels of disturbance, and as such they 30 are highly susceptible to plant invasions. One such invasion that is of major concern to the Great 31Lakes dune systems is that of perennial baby's breath (Gypsophila paniculata). The invasion of 32 baby's breath negatively impacts native species such as the federal threatened Pitcher's thistle 33 (Cirsium pitcheri) that occupy the open sand habitat of the Michigan dune system. Our research 34 goals were to (1) quantify the genetic diversity of invasive baby's breath populations in the 35Michigan dune system, and (2) estimate the genetic structure of these invasive populations. We 36 analyzed 12 populations at 14 nuclear and 2 chloroplast microsatellite loci. We found strong 37 genetic structure among populations of baby's breath sampled along Michigan's dunes (global 38 F ST = 0.228), and also among two geographic regions that are separated by the Leelanau 39 peninsula. Pairwise comparisons using the nSSR data among all 12 populations yielded 40 significant F ST values. Results from a Bayesian clustering analysis suggest two main population 41 clusters. Isolation by distance was found over all 12 populations (R = 0.755, P < 0.001) and 42 when only cluster 2 populations were included (R = 0.523, P = 0.030); populations within cluster 43 1 revealed no significant relationship (R = 0.205, P = 0.494). Private nSSR alleles and cpSSR 44 haplotypes within each cluster suggest the possibility of at least two separate introduction events 45 to Michigan. 46 47