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15 We next visited the region 27 November 1976 and seined extensively near Monsanto Well. Each pupfish with an apparently introgressed phenotype was preserved or discarded on the bank and each pupfish that externally appeared to be pure C. bovinus was returned to the creek. The distribution of pupfish phenotypes was associated with water temper- ature. Those individuals with classic C. bovinus appearance were concentra- ted around spring boils (the air temperature averaged 0°C) and the hybrid types were often found in the shallow (and cold) water. Because of the in- clement weather (rain and strong north winds) this selective seining effect was construed to have had only partial success. We visited Leon Creek again on 13 March 1977 en route to sample the saline Rio Grande faunal assemblage (Hubbs, et al., 1977). We made extensive samples from Monsanto Well downstream for 500 meters and from pools near Highway 18. No trace of hybridization was found more than 500 meters from Monsanto Well and contamination in the Cow Pond observed in August 1976 had been eradicated since the pool had dried. As before, pupfish with apparent hybrid phenotypes were killed and those that seemed pure C. bovinus were returned to the stream. Anthony A. Echelle reported in November 1977 that hybrids were present extensively in the Monsanto Well area (and that the two seg- ments were connected). It was felt that a concentrated effort to eradicate the hybrids should be reinitiated. It was planned to treat the lower segment with ichthyocides twice with a two-week interval between treatments. The first treatment was to be with antimycin and the second with rotenone (The application to use toxaphene was denied by the Environmental Protection Agency.). The use of different ichthyocides was to avoid selection I-or ichthyocide specific resistance; the second treatment with rotenone was because rotenone was the primary toxicant used on the first eradication effort. This plan was disapproved by the U. S. Department of Interior Permit Office because it would by necessity have killed many Gambusia nobilis (an officially endangered species, abundant in four areas 100+ kilometers apart) and Cyprinodon bovinus was merely proposed for listing despite its existance in only one area, where it was outnumbered by Gambusia nob.ilis. While efforts were underway to have the application denial reversed, Dr. Echelle reported that he had obtained some fish with a hybrid phenotype from the Diamond-Y Springs outflow. We felt it too hazardous to treat both segments with ichthyocides. He obtained several Notropis lutrensis in the same collection which suggests a bait bucket release as the cause of the new contamination. As an extensive effort had been planned for the April poisoning; those volunteers were diverted (and their numbers expanded) to another selective seining operation. Three to four seines were in constant use during the three-day effort. The first day (21 April) was spent seining the 500 meter segment adjacent to Monsanto Well. The accessible locations were seined until Cyprinodon captures were nil (Much of the marsh area was overgrown with Scirpus, an environment dominated by Gambusia, with the Cyprinodon concentrated in the more open areas.), apparently pure C. bovinus were set aside and the fish with hybrid phenotypes were discarded. This process was done twice. On the second day the area around Diamond-Y Springs was extensively sampled but no fish with obvious hybrid phenotypes were obtained. A few that were remotely suspect were discarded. It seems that Dr. and Mrs. Echelle were so vigorous in their effort to document the