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1 16 occurrence of hybrid phenotypes in the Diamond-Y outflow that they eradicated the contamination. We regret that Dr. Suttkus had not had similar results when demonstrating the presence of C. variegatus in the creek near the High- way 18 bridge. That afternoon we repeated the sampling effort (three times) in the Monsanto Well area and examined the circumstances downstream. The downstream locations showed no contamination except for the pool at the Highway crossing where a few fish with possible hybrid phenotypes were eliminated. The third day effort was concentrated in the Monsanto Well area (two times) and a sample from the pool by Highway 18 contained no suspected hybrids. Our efforts at Monsanto Well clearly were less productive on successive days and few possibly contaminated fish were found by the third day. We returned 11 August, 1978 to assess the situation. We found no obvious hybrid phenotypes but again eliminated those few (ca 2%) within the C. bovinus phenotype but at the edge of its phenotypic variation most closely approaching C. variegatus. Variation within the sample was minimal and apparently the C. bovinus phenotype prevailed in the entire water course. The phenotypic extremes, hybrid and putative C. bovinus, occupied different habitats during our sampling, eurythermal and quiet water vs stenothermal and current, respectively. It is possible that natural selection was a major contributant to the rever- sion to the C. bovinus phenotype. Predation by the centrarchids, Lepomis cyanellus and Micropterus salmoides may further refine that selection as young of both species (with fish in their guts) were found near Monsanto Well in our August 1978 samples. On occasion, extensive flow from the upper segment enters the Monsanto Well area. A large pool (or dry hole) is at the upper end of the Monsanto Well region. This pool results from water flowing over a clay bank creating a 0.3 meter high cataract when the pool is full. In August 1978 that pool had a typical Monsanto Well ichthyofauna but the creek immediately above had a vastly different assemblage dominated by Fundulus. Extensive efforts produced no Cyprinodon. Apparently, Fundulus could swim up over the cataract but not Cyprinodon. The Echelles made similar observations the previous July. It is therefore unlikely that lower Leon Creek pupfish are able to populate the upper segment in sub- stantial numbers. Downstream gene flow seems likely (as occurred with Lepomis and Micropterus). The homogenous Cyprinodon in Leon Creek could have resulted from selection (either ours or natural) for a C. bovinus like genome. Simi- larly, a breeding system with sexual preference for novel individuals would tend to reduce chance diversity. Loiselle's demonstration of preference for Cyprinodon nevadensis or C. macularius males over C. bovinus males by C. bovinus females would, if extended to C. variegatus males, result in substantial hybridization (= Dr. Echelle's November 1975 observations). Any hybrid hatched near the highway bridge that dispersed away from the bridge would then be a novel individual in that new environment and again have a mate selection advantage. Eventually the dispersants would reach the two ends of the stream and no longer be able to enter a population not previously exposed to any C. variegatus phenotype (no matter how dilute). Subsequently mating of opposites would result in uniformity in successive generations. Each unique fish would most often mate with the other extreme or in effect produce an intermediate. Random mating would occasionally result in pairing