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initial diversion and the location of the 1938 fish sample. The area covered by Lake Leon in 1971 (the date for the map traced in Fig. 1) was probably less than that when the lake was full as the "level" mapped is 1 foot lower than the eroded and settled spillway. Presumably all spring heads were originally inundated. Water use reports for the lake indicate Lake Leon was full until 1932 (6 annual reports), had seasonally deficient supplies from 1939 until 1946, and that the source for irrigation water included artesian supplies consistently until 1946, alternated between creek or lake water and spring or artesian water from 1947 to 1951, and subsequently, the irrigation water was primarily or exclusively pumped ground water. De- spite the 'change from artesian flow to pumped water, water use also suggested a decline in supply, above 5000 acre foot/year until 1932, above 4000 acre foot/year until 1944, above 3000 acre foot/year until 1960 and use was as low as 1000 acre foot in 1971. At the present, Lake Leon is formed from water pumped from wells and stored for gravity flow irriga- tion. The surrounding land is planned for development into a residential subdivision. W. L. Minckley and W. E. Barber collected a pupfish from "Willbank Spring," north of Fort Stockton in December 1965 (Minckley and Arnold, 1969). The property at that time was owned by Henry Wilb.ank but the spring is properly ,,named Diamond-Y Springs (Brune, 1975). Its location 310 N, 102 54' W clearly does not correspond to that recorded by Lt. Whipple for Leon Springs and its elevation is lower than Fort Stockton, thus in contrast with the higher (1000 ft.) elevation of Leon Springs compared with Comanche Springs on Graham's (1852) elevation profile. Minckley thought the newly located population might be C. bovinus, a tentative identification confirmed by Echelle and Miller (1974). Hubbs and Echelle (1973) had previously dis- cussed Cyprinodon bovinus as extant and endangered. Prior to the water depletion that accompanied extensive irrigation in the Leon Creek valley it is likely that water flowed from Leon Springs to that segment of Leon Creek adjacent to Diamond-Y Springs (if not permanently, at least with sufficient frequency that the population would have occasional genic interchange). Echelle and Miller (1974) report that the six remaining (of 16) syntypes (except for deterioration due to the age of the material) match well the attributes of pupfish recently collected from Leon Creek. As Echelle and Miller (1974) point out, the entire range is in the mid- dle of an oil and gas field and Diamond-Y Springs is less than one kilometer north (and down slope) from the Gomez (gas cracking) Plant of Northern Natural Gas Company. The oil field was developed in the 1940's (Ross L. Shipman, pers. comm.) and the pupfish have survived oil (and later gas) well drilling as well as more than 30 years of oil field operation, during which time a contamination sufficient to exterminate the fish did not occur before the fish was found to occur in the creek. Officials of Northern Natural Gas Company and Exxon Corporation (a primary participant in the oil field) have been receptive to ecological recommendations. In May 1974 the Soil Conservation Service built an earth dike to protect Diamond-Y Springs from possible problems that might occur in the Gomez Plant. Existing permanent water inhabited by Leon Springs pupfish occurs in two semi-isolated segments about 15 km north of Fort Stockton (Fig. 2). The most pronounced spring source is Diamond-Y Springs which is located 500 meters north of the Gomez Plant. Water flows NE from Diamond-Y Springs for about 1 kilometer where it joins the main channel of Leon Creek. Leon Creek originates about 1 km to the west of the junction in