The Pennsylvanian Naco Formation in the southeastern Dripping Spring Mountains near Winkelman, Arizona, consists of a lower dolomite member and an upper limestone member. A strip approximately one mile wide across the mountains was mapped and two stratigraphic sections of 405 and 538 feet were measured and are described in detail. The formation contains an abundant and varied fauna of Derryan and Desmoinesian age which is described and illustrated.