The concern caused by the explosive spread of the zebra mussel, (Dreissena polymorpha), within the United States resulted in passage of the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 (P.L. 101-646). The impact of this biofouling pest on the economy and ecological processes in the U.S. and Canada has required prompt action on a large scale to prevent further infestations and minimize ecological degradation. This map layer is a compilation of reports of confirmed zebra mussel sightings in the United States from 1988 to January 2008. It provides geographical and historical information to show distribution over time. The reports came from a variety of Federal, State, and municipal agencies, public utilities, universities, engineering and private consultant firms. The locations of confirmed sightings were registered at 1:100,000-scale on EPA Reach File Version 3.0 and are maintained as an ArcInfo export file. This is a revised version of the June 2005 map layer.