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JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 20 como especies en vias de extincion, vulnerables o en un estado indeterminado. De estas especies, 46 se han catalogado en vias de extincion. Ademas, 18 especies de peces se han registrado recientemente como extinctos. Se ha determinado que los ecosistemas de esas especies en vias de extincion son 15. De ellos, los mas significativos son: Cuatro Cienegas (Coahuila), el Rio Gila (Nuevo Mexico, Arizona y Sonora), el Rio Bravo del Norte (Nuevo Mexico, Texas, Chihuahua, Coahuila, Nuevo Leon y Tamaulipas), el Rio Pecos (Nuevo Mexico y Texas), el Valle Railroad (Nevada), el Rio Colorado (Colorado, Utah, Nuevo Mexico, Arizona, Nevada, California, Baja California del Norte y Sonora), el Rio Green (Wyoming, Colorado y Utah), el Valle Pahranagat (Nevada), el Valle de Parras (Coahuila), La Media Luna (San Luis Potosi), Ash Meadows (Nevada), el alto Rio White (Nevada), el Rio Moapa (Nevada), el Rio Yaqui (Arizona, Chihuahua y Sonora) y la cuenca alta del Rio Klamath (Oregon y California). Se discuten esos ecosistemas y sus especies de peces, anfibios, reptils y invertebrados en vias de extin- cion. La proteccion de los habitats naturales que permanecen dentro de estas areas provee la major via a la conservacion a largo organismos raros. INTRODUCTION.-During the past century, physical and chemical alteration of habitats and the introduction of exotic species have caused declines of many native aquatic organisms in North American deserts. Despite a depau- perate and relatively poorly-known ichthyofauna, 39 fishes from the desert areas of the United States have been listed by the Department of the Interior as endangered or threat- ened (Williams 1981). Deacon et al. (1979) compiled a more comprehensive list of fishes, endangered, threatened or of special concern, from the United States, Canada and Mexico that included 139 desert fishes from North Amer- ica. Environmental perturbations have now reached the level that some desert fishes are extinct and entire aquatic ecosystems have been eliminated. These losses are especially disconcerting because most aquatic organisms inhabiting waters of North American deserts are not well-known. This is particularly true for Mexican fish species as well as for invertebrates, both in the United States and Mexico. Many taxa are undescribed. Additional ecological, taxonomic and systematic work, therefore, is sorely needed. Although the general decline of aquatic ecosystems in North American deserts has been known for some time (e.g., Contreras-Balderas 1978, Miller 1961, Minckley and Deacon 1968, Pister 1981), this decline has not been quantified other than to develop lists of vanishing species. The primary purpose of this paper is to determine the most significant aquatic ecosystems in North American deserts that are endangered and to discuss their importance in terms of remaining natural areas and constituent aquatic organisms. The continued existence of all aquatic eco- systems in North American deserts is important regardless of their size or diversity. However, it is our intent to focus on those areas that contain large numbers of endangered, threatened or otherwise rare aquatic organisms. In this manner, the need to take immediate measures to preserve these ecosystems becomes more apparent. A second pur- pose of this paper is to present an updated list of rare fishes from North American deserts. The information presented herein should be revised and expanded with increased knowledge of these aquatic organisms. METHODS.-The geographic area considered in this paper corresponds roughly with the limits of the Great Basin, Mohave, Sonoran and Chihuahuan Deserts (see Fig. 1). Additionally, we have included certain basins adjacent to these desert areas, such as the Upper Colorado River Basin, Upper Klamath River Basin, Upper Pit River Basin and Malheur Basin. Thus within the United States, we include parts of Oregon (OR), California (CA), Wyoming (WY), Texas (TX) and Colorado (CO), and all of Nevada (NV), Utah (UT), Arizona (AZ) and New Mexico (NM). Within Mexico, we include parts of Sinaloa (SIN), Durango (DGO), Zacatecas (ZAC) and San Luis Potosi (SLP), and all of Baja California (BCA), Baja California Sur (BCS), Sonora (SON), Chihuahua (CHI), Coahuila (COA), Nuevo Leon (NLE) and Tamaulipas (TAM). In delineating the boundary of individual ecosystems, we utilized the distribution of native fishes in the region. Most basins or valleys form easily definable ecosystems because they contain a unique assemblage of native fishes that often is endemic. For example, Ash Meadows con- stitutes a natural ecosystem because its drainage is endor- heic (i.e., internal) and because it harbors a series of en- demic fishes; the Ash Meadows speckled dace, Devils Hole pupfish, Ash Meadows Amargosa pupfish, warm springs Amargosa pupfish and Ash Meadows poolfish. However, some ecosystems are less easily defined. For example, we consider the Rio Grande as a single ecosystem, and con- sider each major tributary (e.g., Rio Conchos and Pecos River) separately. The use of fish distribution to determine ecosystem boundaries is often considered a valid method- ology when working with aquatic systems (Hawkes 1975). For an ecosystem to be included herein, it must have pro- vided habitat for at least four rare fishes during recent times. To indicate the status of individual taxa, we utilize the following categories as defined by the International Union for Conservation of Nature and Natural Resources (IUCN): endangered (E) - taxa in danger of extinction and whose survival is unlikely if the causal factors continue operating; vulnerable (V) - taxa believed likely to move into the endan- gered category in the near future if the causal factors con- tinue operating; rare (R) - taxa with small world popula- tions that are not at present endangered or vulnerable, but are at risk; indeterminate (1) - taxa that are suspected of belonging to one of the above three categories but for which insufficient information is currently available. All designations given to the aquatic organisms listed herein are those of the Desert Fishes Council's Endangered Species