Pathways of introduction can have important effects on the extent and rate of the invasion of a species. However, information about invasion history is often difficult to obtain. Here, I examined the source of the aquatic plant ' Cabomba caroliniana' (fanwort) in Canada. The species is native to the southern United States and South America and has spread worldwide, including to the northern United States and now southern Canada (Kasshabog Lake, Ontario). I sampled plants from 20 populations in the North American native and introduced range and from commercial suppliers and used analyses of DNA content and plastid gene sequences to test the hypothesis that Canadian plants were introduced via the aquarium trade. Contrary to common assumption, populations in the northern United States are a more likely source of Canadian plants than commercial sources. It is unknown whether the plants arrived naturally or as a result of human-mediated boat traffic.