A stratigraphic and faunal study of the Wolfcampian Series (lower Permian) of the Glass Mountains, west Texas, the standard section for North America, establishes a fusulinid zonation for this important series. The Wolfcampian Series, deposited in near-shore environments on the northern flanks of the Marathon orogenic belt, is a complex set of facies having many unconformities. The series is divided into two formations separated by a major unconformity (Ross, 1959a): the lower one (the upper part of the Wolfcamp Formation of Udden, 1917), the Neal Ranch Formation, and the upper one (the Wolfcamp Formation of King, 1931, in the western Glass Mountains), the Lenox Hills Formation. These formations contain distinctly different species of fusulinids but both formations are within the zone of Pseudoschwagerina. The base of the series is raised to exclude strata of late Pennsylvanian age ( i.e., the Uddenites- bearing shale member and bed 2 of the gray limestone member, King, 1931), and the top is raised in the eastern Glass Mountains to include strata previously placed in the lower part of the Leonardian Series. Fusulinid assemblages from the upper Pennsylvanian Gaptank Formation, the Wolfcampian Series (lower Permian), and the lower part of the Leonardian Series (lower middle Permian) include 50 species collected in 43 measured stratigraphic sections and from more than 260 localities. The distribution of the different species is closely associated with rock types. The fusulinid assemblages indicate that bed 2 of the gray limestone member (Gaptank Formation) is probably correlative with the upper part of the Virgilian Series of Kansas, although its upper part may be slightly younger and possibly equivalent to the lower parts of the Pueblo Group of Texas and the Admire Group of Kansas and to portions of the upper part of the Orenburgian Series of Russia. The Neal Ranch Formation is apparently correlative with the lower part of the Asselian Series of Russia, the Council Grove Group of Kansas, part of the Earp Formation of Arizona, and part of the Bird Spring Group of California and Nevada. The Lenox Hills Formation probably correlates with the upper part of the Asselian Series of Russia, most of the Hueco Limestone of west Texas, the Moran, Putnam, and Admiral Formations of north-central Texas, the Chase Group of Kansas, and most of the McCloud Formation of California. The lower part of the Leonardian Series contains fusulinids which are indicative of a correlation with the Tastuba and Sterlitimak stages (Sakmarian Series of Ruzhencev, 1956) of Russia, the Bone Spring Limestone of west Texas, and the Arcturus Formation of eastern Nevada.