In 1983, the U.S. Geological Survey began a multiphase study to help define the hydrologic system of the Mesilla Basin in Dona Ana County, New Mexico, and El Paso County, Texas. This report is a compilation of selected geohydrologic data collected through July 1985. The report describes the groundwater monitoring network of 143 wells, including 3 hydrologic sections constructed across the Mesilla Valley. Geohydrologic data presented in the report include: well records and water levels from 143 wells, chemical analyses of water samples from 34 observation wells, borehole-geophysical logs of 10 observation wells, and river stage at 3 hydrologic sections. (USGS)