Hyrilla (Esthwaite Waterweed or Hydrilla) is an aquatic plant genus, usually treated as containing just one species, Hydrilla verticillata, though some botanists divide it into several species. Synonyms include H. asiatica, H. japonica, H. lithuanica, and H. ovalifolica. It is native to the cool and warm waters of the Old World in Asia, Europe, Africa and Australia, with a sparse, scattered distribution; in Europe, it is reported from Ireland, Great Britain, Germany, and the Baltic States, and in Australia from Northern Territory, Queensland, and New South Wales.