Hydrilla is likely the most problematic aquatic weed in the United States. The prolific growth of this weed clogs waterways and costs millions of dollars annually in control efforts. This study is a collaboration with researchers from the University of Florida, ICIPE (International Center of Insect Physiology and Ecology) in Kenya, and the U.S. Army Corp. of Engineers. We are using molecular markers to determine the genetic diversity, clonality, and origins of hydrilla in the United States and a population in the East African Great Lakes. We are also studying variation in the phytoene desaturase (pds) gene which produces an enzyme important in the process of photosynthesis. This enzyme is inhibited by the herbicide fluridone resulting in photobleaching and death of the plant. Some hydrilla populations in Florida have evolved resistance through point mutations in the pds gene; however, the range of resistance levels exceeds the few described mutations. We are asking if other mutations may be associated with resistance or if copy number due to polyploidy may play a role in resistance. Additionally, we are developing molecular markers that will allow rapid screening for herbicide resistance in hydrilla infestations.