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Changing Fish Fauna of the Southwest 393 threadfin shad (Dorosoma petenenw), has failed to reveal native fishes other than Xyrauchen texanus (letter of January 6, 1958, from J. B. Kimsey to Carl L. Hubbs). Both Ptychocheilus lucius and Gila robusta elegans may still be caught in sections of the Colorado River above Grand Canyon, but the series of dams now under construction there may well mean extermination for these fishes in the not too distant future. The Conchos pupfish, Cyprinodou eximius Girard, was described from Chihuahua City, Chihuahua, Mexico, in 1859. On May 29,1903, S. E. Meek collected it in the independent drainage of the Rio Sauz at Sauz, about 30 miles northward from Chihuahua City. Testimony obtained by Clark Hubbs during the summer of 1954 indicates that the Rio Sauz went dry in 1947 and that "there is no living water any- where in the valley" (letter of July C 1954, from Clark Hubbs to Carl L. Hubbs). Whatever local attributes this population may have developed during its isolation from the main range of the species will have to be determined solely from the preserved material. Arivaca Creek, a small spring-fed stream that rises just above Arivaca, southern Arizona (Fig. 1), was originally fishless, according to Fred C. Noon (contacted on April 15, 1950, at Oro Blanco Ranch). It was stocked by the Arizona State Health Department with Gam- busia during a malaria outbreak about. 1936. Thorough examination in 1950 of the main part of the creek (about 175 yards long), above and below the road crossing, revealed only the Gila topminnow (Poeciliopsis occidentalis), which may have been the fish introduced as "Gambusia" about 1936, reportedly from the vicinity of Tucson. It was stated that catfish and bass had been planted on the flat above Arivaca, but these species were unknown in the creek. The native topminnow was again found to be common there on April 12, 1957, when I secured a live stock for experimental work. On March 30, 1959, however, when the stream was revisited to replenish our live material, Poeciliopsis had been completely replaced by the exotic mosquitofish, G. affinis a ffinis. How and when the latter species gained entry to Arivaca Creek is not known, but P. occidentalis was replaced by the more competitive alien in a rather short time-apparently in less than 2 years. Charles H. Lowe, Jr_, of the University of Arizona, visited the creek in June, 1959, expresdy to check my finding and also was unable to locate a single specimen of Poeciliopsis. The introduc- tion of G. afftinis in other parts of the Southwest has led to the reduc-