Three polymorphic enzyme loci were used to study mating system and genetic structure in a disjunct population of the seaside goldenrod, Solidago sempervirens, growing near Windsor, Ontario to determine if self-fertility played a role in the initial colonization of this area. Observations on genotypic variation in the adult populations and the analysis of progeny arrays showed no evidence for inbreeding with estimates of outcrossing rate not significantly different from t = 1·00 for both the Windsor population and a population from the main distribution of the species. Colonization of southern Ontario by S. sempervirens probably involved either a single colonization event consisting of several individuals with different self-incompatibility genotypes or a number of separate colonizing episodes.