Abstract The recent recognition of invasive hybrid watermilfoil (Myriophyllum spica- tum · M. sibiricum) in North America has neces- sitated a more thorough evaluation of its overall distribution and occurrence in natural popula- tions. A comprehensive survey of watermilfoil populations was conducted in five Minnesota lakes, three of which were suspected a priori to contain hybrid watermilfoil. DNA sequence data verified that hybrid plants between the nonindig- enous M. spicatum L. and indigenous M. sibiri- cum Kom. occurred in three of the five lakes sampled. Myriophyllum spicatum was not de- tected in lakes where hybrids were prevalent. Further sampling of lakes in Idaho, Michigan, Minnesota, Wisconsin and Washington identified 30 additional hybrid watermilfoil populations. In only three of these populations the hybrid water- milfoil was found to co-occur with M. spicatum. To facilitate the field identification of the two parental species and their hybrid, morphological data from watermilfoil specimens collected across the United States were evaluated. We determined that leaf segment/leaf length measurements can effectively distinguish M. spicatum and M. sibir- icum; however, hybrids are intermediate for these characters and such measurements frequently overlap with respect to their parental taxa. By incorporating a combined molecular and mor- phological approach to identifying watermilfoils, the hybrids can be identified readily and their distributions elucidated both within and between lakes. Because hybrids may respond differently to local ecological conditions than their parents, information on their presence and distribution should be of particular importance to manage- ment and conservation programs. Keywords Aquatic plants Æ Geographic distribution Æ Hybrid Æ Invasive Æ ITS Æ Myriophyllum spicatum Introduction The nonindigenous Eurasian watermilfoil (Myr- iophyllum spicatum L.) is widely recognized as a problematic invasive plant in North America, primarily due to the rampant spread of this aquatic species throughout waterbodies of 46 states and three Canadian provinces (Jacono M. L. Moody Æ D. H. Les Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269-3043, USA Present Address: M. L. Moody (&) Department of Biology, Indiana University, Jordan Hall 142, 1001 E. 3rd St., Bloomington, IN 47405, USA e-mail: miclmood@indiana.edu Biol Invasions (2007) 9:559–570 DOI 10.1007/s10530-006-9058-9 123 ORIGINAL PAPER Geographic distribution and genotypic composition of invasive hybrid watermilfoil (Myriophyllum spicatum · M. sibiricum) populations in North America M. L. Moody Æ D. H. Les Received: 3 January 2006 / Accepted: 21 September 2006 / Published online: 9 January 2007 Ó Springer Science+Business Media B.V. 2006