Upper Paleozoic sedimentary rocks are widely distributed and probably more fully represented in the Peruvian Andes than in other parts of South America. A reconnaissance study of these rocks and their fossils reported here supplies new and important information on late Paleozoic history of the western part of South America. Continental plant-bearing strata of Mississippian age are described, and their known distribution is indicated. Great sequences of marine Pennsylvanian and Permian strata were studied over a large area. The Pennsylvanian is represented mainly by the Atokan stage, lower middle Pennsylvanian, and the Permian belongs chiefly to the Wolfcampian, or lower Permian. Limited areas of middle Permian, Leonardian rocks are also reported. There is no known evidence for glaciation during the late Paleozoic in Peru. The earliest volcanic activity in the Peruvian Andes probably took place during the Mississippian period as suggested by tuffaceous rocks and andesite pebbles in the Mississippian Ambo group. The entire region was remarkably free from orogeny between the early Devonian and the Permian, but local warping occurred at several times and places. Most of the stratigraphic breaks in this interval are recognized mainly by paleontological evidence. Locally, slight angularities characterize unconformable contacts, but more commonly the unconformable beds are nearly parallel. The most pronounced stratigraphic break in the upper Paleozoic lies well up in the Permian, and this undoubtedly records one of the more significant episodes in Andean history. Marine lower and middle Permian strata are overlain unconformably by an enormous and very extensive series of red, coarse arkosic clastics and volcanics belonging to a higher stage of the Permian system. The distribution of these younger rocks and the manner in which they overlap older rocks toward the western ranges suggest that the Permian orogeny and volcanism were most intense in the area which now is coastal Peru. This is the earliest well-defined orogeny in this part of the Andes. Prior to the Permian orogeny there is no satisfactory evidence of a western source of sediments. The marine invertebrates of the Pennsylvanian show affinities both with Amazonian Pennsylvanian in Brazil and with middle Pennsylvanian faunas of the southwestern United States. Likewise the Permian faunas are similar to those of the southwestern United States. Essentially they are like faunas of the same age in Bolivia. Probably they ultimately will be recognized in Ecuador, Colombia, and Venezuela when the upper Paleozoic faunas of the Northern Andes are better known. General similarity between Peruvian and Texas lower Permian faunas suggests that climatic or other barriers to migration between the two areas were ineffective. Probably the submarine climates of the two areas were similar. Six well-defined paleontological zones are delineated, one in the Mississippian, one in the middle Pennsylvanian (Atokan), three in the lower Permian (Wolf-campian), and one in the middle Permian (Leonardian). The invertebrate faunas are described and figured. These include 180 forms of which 42 are left without specific assignment. Three genera are erected—two bryozoans and one gastropod. Corals are under study by Robert Finks.