The dioecious plant Egeria densa Planchon (Hydrocharitaceae), indigenous to fresh water habitats in Brazil, Uruguay, and Argentina, has been introduced to many temperate habitats throughout the world where it propagates clonally and often becomes a serious weed. Representatives from populations in Oregon (USA) and southern Chile were tested for genetic variability by random amplified polymorphic DNA (RAPD) fingerprinting. Little genetic variability among samples was found despite the extreme geographical distance between the two introduced populations. These results suggest that similar bottlenecking events impacted both introductions or that there is low genetic diversity within the native source populations. Surveys of the genetic diversity of plants within the native range and more widespread and descriptive surveys of introduced populations are necessary to clarify the significance of the genetic similarity of these two widely separated populations