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account for the inability of C. baileyi from Preston Spring to survive the more poorly oxygenated waters of Mormon Spring by comparing the gill filament number and length in fishes from the two springs. Insignificant differences existed in gill filament number; however, gill filament length varied significantly. Of 10 C. baileyi from Mormon Spring (27.8-37.1 mm), the mean value of longest gill filament was .046 SL (range .040-054). Ten C. baileyi from Preston Big Spring (32.6-40.7 mm) exhibited a mean longest gill filament value of .0362 SL (.034-.042). Variation in gill surface area is apparently attributable to evolution of C. baileyi populations in springs with different dissolved oxygen concentrations. Deacon and Minckley (1974) attributed the survival of C. baileyi in poorly oxygenated water to their proportionately longer gills as compared to other desert fishes. Both geographic isolation and environmental factors are important considerations in designating the five subspecies herein described. Crenichthys b. albivallis occurs in the seven northernmost springs inhabited by the species. These springs are characterized by cool temperatures (near 20oC) and relatively high dissolved oxygen at the spring source (above 3 ppm). Crenichthys b. thermophilus inhabits three springs near a dry tributary of the pluvial White River approximately 20 km southwest of Lund Town Spring. The springs inhabited by C. b. thermophilus are warm (33.3-37.0oC) and have low dissolved oxygen at the spring source (0.7-2.0 ppm). Over 100 km south of these springs are the springs of Pahranagat Valley and the headwaters of the Moapa River. The Ash Spring population has distinct meristic features, and is therefore considered separable from other C. baileyi populations. Other Pahranagat Valley populations, Hiko and Crystal, are considered distinct from Moapa River populations due to geographic isolation and differences in color. The authors are indebted to the many people who provided assistance throughout the various stages of this study. Particularly, Carl L. Hubbs and Robert R. Miller deserve acknowledgment for the loan of museum specimens, field notes, radiographs, and their extensive knowledge of these fishes. William N. Eschmeyer kindly loaned the remaining type specimens of Crenichthys baileyi. James E. Deacpn provided valuable assistance throughout this work and reviewed the manuscript. Clark Hubbs provided a beneficial review of an earlier draft of this paper. Thorn B. Hardy kindly provided location data and status reports for several populations. Thanks are due Carl E. Bond for allowing one of us aW) to work on something besides Gila. Cynthia D. Williams typed and reviewed drafts of this paper. LITERATURE CITED BAILEY, R. M., J. E. FITCH, E. S. HERALD, E. A. LACHNER, C. C. LINDSEY, C. R. ROBINS, and W. B. SCOTI. 1970. A list of common and scientific names offishes from the United States and Canada. 3rd ed. Am. Fish. Soc. Spec. Publ. 6, 149 pp. BLACKWELDER, E. 1934. Origin of the Colorado River. Bull. Geol. Soc. Am. 45:551-556. CROSS, J. N. 1976. Status of the native fish fauna of the Moapa River (Clark County, Nevada). Trans. Am. Fish. Soc. 105:503-508. DEACON, J. E., and W. G. BRADLEY. 1972. Ecological distribution of the fishes of the Moapa (Muddy) River in Clark County, Nevada. Trans. Am. Fish. Soc. 101:408-419. DEACON, J. E., and W. L. MINCKLEY. 1974. Desert fishes. In Desert Biology II (G. W. Brown, Jr., ed.), Academic Press Inc., New York. DEACON, J. E., and B. L. WILSON. 1967. Daily activity cycles of Crenichthys baileyi, a fish endemic to Nevada. Southwest. Nat. 12:31-44. DEACON, J. E., C. HUBBS, and B. J. ZAHURANEC. 1964. Some effects of introduced fishes on the native fish fauna of southern Nevada. Copeia 1964:384-388. GILBERT, C. H. 1893. Report on the fishes of the Death Valley expedition collected in southern California and Nevada in 1891, with descriptions of new species. N. Am. Fauna 7:229-234. 502 .