A small mollusk called the zebra mussel has been steadily invading America's rivers and lakes. Zebra mussels colonize water intake pipes and severely restrict the water flow. This has cost billions of dollars of damage to municipal and private facilities that rely on fresh water. This map layer shows the locations of confirmed zebra mussel sightings in the United States from 1988 to 1998. The sighting reports came from a variety of federal, State, and municipal agencies, public utilities, universities, engineering and private consultant firms, and businesses. 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. 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 data set is a compilation of reports of confirmed zebra mussel sightings in the United States from 1988 to the present. 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 Arc/INFO export file. This is a revised version of the June, 2000 data set. The data is an essential part of the Department of the Interior's Nonindigenous Aquatic Nuisance Species Program. The data was compiled for use by resource managers to make them aware of zebra mussel occurrences so that they may help to control or prevent the spread of zebra mussels. These data are intended for geographic display and analysis at the national level, and for large regional areas. The data should be displayed and analyzed at scales appropriate for 1:2,000,000-scale data. No responsibility is assumed by the U.S. Geological Survey in the use of these data.