Lippia ('Phyla canescens' (Kunth) Greene: Verbenaceae) is a very invasive weed in wetlands and riparian zones in Australia and especially in the Murray Darling Basin. The use of chemicals as a control method is inappropriate in such environmentally sensitive areas, particularly the Macquarie Marshes and Gwydir Wetlands. Biological control will be the only option in many areas. We investigated genetic diversity in 12 populations of lippia from four different catchments in Australia, eight populations from the native range in South America and five populations from France where the species is non-native and invasive. Low levels of genetic diversity were detected within some Australian regions in contrast to the Argentinean and French populations. In the analyses the Australian material segregated with two disjunct regions in Argentina, suggesting that Australia has experienced multiple introductions of lippia.