The abundant and well-studied Early Permian faunas of the eastern Klamath terrane, which contain a high percentage of endemic species and genera, constitute part of a biotic province (the McCloud province) that is distinct from the better known central Cordilleran and Tethyan provinces. The McCloud province evidently fringed an island arc that was located within the tropical climatic zone of the Paleopacific Ocean during the Early Permian. The development of an endemic biota in this province required considerable isolation for long periods of time, and because the shallow-water biotic provinces of the eastern Klamath terrane, Tethys, and the central Cordillera lay along the same general latitudinal band and were open to waters of the same ocean basin, their separate development must have been the result of geographic barriers. We conclude that geographic isolation was produced by large distances of deep water that lay between these shallow-marine provinces in the Early Permian Paleopacific Ocean. By analogy with biotic distributions in modern ocean basins, these deep-water barriers must have been several thousand kilometers wide.