The potentiometric surface of water in the lower Cretaceous Lakota Formation and equivalent units (Cut Bank Sandstone Member , Sunburst Sandstone Member, and Moulton Member of Kootenai formation; Third Cat Creek sandstone of Kootenai Formation; Pryor Conglomerate Member of Kootenai Formation) is shown on a base at a scale of 1:1,000,000. The map is one of a series produced as part of a regional study of aquifers of Cenozoic and Mesozoic age in the northern Great Plains of Montana. The contour interval is 200 feet. Water in the Lakota Formation and equivalent units occurs under water-table and artesian conditions. The direction of regional groundwater movement is east and north from the recharge areas in the Little Belt and Big Snowy Mountains. The average discharge from 20 wells is about 29 gallons per minute and the specific capacity of 18 wells averages 0.75 gallon per minute per foot of drawdown.