A map showing the net change in the poentiometric surface of the Magothy aquifer in the Magothy Formation of Cretaceous age in southern Maryland from September 1975 to September 1995 was based on water-level measurements in 67 wells. The map shows that the decline of the potentiometric surface during the 20-year period ranged from 2 to 21 feet in the northernmost part of the study area. The decline was greater than 40 feet in parts of southern Prince Georges County, 75 feet at Waldorf, and 28 feet at the Chalk Point powerplant.