This map shows the locations of the 136 observation wells from the observation-well network maintained by the U.S. Geological Survey in Massachusetts and Rhode Island. The wells are identified by town name and well number. The map shows the location of the 10 observation wells that have digital recorders and the 126 observation wells that are measured by local observers. The aquifer material (sand, till, or bedrock) in which a well is located is noted. County and town boundaries are shown on the map. These features are presented at a scale of 1:400,000 (map size is about 38 by 30 inches). The map includes textual information describing the uses of observation-well data. The information is organized by construction, water supply, water quality, and statistical analysis. The map also presents observation well information, which was obtained from the annual data report of the Massachusetts--Rhode Island District. This infor- mation is presented in tabular form and includes town name, well number, aquifer material in which the well is located, and well depth below the land surface. The map was produced from a digital data base using a Geographic Information System. State boundaries were generated from digital line graphs maintained by the U.S. Geological Survey. Town and county boundaries were digitized from stable-base materials maintained by State agencies. The map was prepared in cooperation with State agencies of Massachusetts and Rhode Island.