The statewide observation-well network of the U.S. Geological Survey was developed in Montana in 1947 to record long-term fluctuations of water levels in selected wells. This report was prepared to summarize well records and water levels for the network and to show the location and density of the wells. Data for 299 wells are tabulated in this report, generally within the interval 1947-85. The well records include altitude of land surface, date drilled, well depth, use of site, use of water, casing type, casing size, casing depth, top and bottom of perforations, principal aquifer, year water-level measurements began and measurement frequency, year chemical analysis began, type of analysis, analyzing agency, and type of logs available. The water-level data include date of measurement and depth to water below land surface. Well location and density are shown on a map at a scale of 1:1,000,000. (USGS)