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14 in recommendations to eradicate the hybrid swarm (Kennedy, 1977). The area was examined by Dr. Echelle and Texas Parks and Wildlife personnel in January 1976 to determine the extent of the hybrid swarm. They found hybrids in the entire lower segment from Monsanto Well to the Cow Pond but some segregation of C. bovinus phenotypes in flowing water and C. variegatus phenotypes in quiet water (A.A. Echelle, pers. comm.). A stock of C. bovinus was flown,to the Federal Fish Hatchery in Dexter, New Mexico, by the Texas Parks and Wildlife airplane as insurance against a possible disaster. The lower segment of Leon Creek was treated with rotenone on 13 February 1976. Prior to the rotenone treatment six segments (up to 100 meters in length) were seined and all fish (except Cyprinodon) and macroinvertebrates set aside and reintroduced into the springs 500 meters ENE of Monsanto Well on 14 February (Hubbs,et al., 1978). On that date more than 100 C. bovinus and associated indigenous fishes were obtained from Leon Creek 350 meters NE of the junction with the Diamond-Y Springs outflow and were also released with the downstream samples. On 28 February 300+ more C. bovinus from the same pool and 100+ C. bovinus, from the Diamond-Y Spring Run, associated fishes (except for Lepomis cyanellus as we wished to avoid excessive predation when its prey fish species populations were at minimal levels), and macroinvertebrates were released at Monsanto Well, the spring site 500 meters ENE, and at the Highway 18 bridge. On 13 February the rotenone was initially released in the Cow Pond and the entire creek and adjacent marshes sprayed, downstream to upstream; by midday when the sgray crews reached the Highway 18 bridge, the weather had warmed from near 0 C and courtship activity was noted in a pool upstream from the bridge. That spot was also treated with antimycin to kill any eggs that might have been spawned. Rotenone drip bottles were placed over the springs near Monsanto Well and those 500 meters to the ENE. The upper pool at Monsanto Well was so heavily treated with rotenone that the milky appearance of its water persisted until 28 February. During that visit the lower segment of Leon Creek was examined on foot and the only Cyprinodon (or any other fish) observed were near the two-week old release location. Subsequently, the other fishes were obtained and released. Leon Creek was next examined 6 August, 1976 (Hubbs, et al., 1978) and samples were obtained from the Cow Pond, Highway 18 bridge, and Monsanto Well. All released fish species except G. geiseri were found in our collections. The hybrid swarm between G. nobilis and G. affinis was re- established (Hubbs, et al., 1978) and has subsequently resumed pretreat- ment conditions. Both Lucania and Fundulus are now quite abundant and the 1976 population decline was only temporary. Traces of Cyprinodon genic contamination were found in adults and young from the Monsanto Well sample (the highway bridge sample seemed uncontaminated but one large female with a hybrid phenotype was collected from the Cow Pond). The obvious failure to eradicate the hybrids was apparently concentrated near Monsanto Well. That location had been exposed to a substantial superlethal dose of rotenone but some adult hybrids must have survived. Our suspicion that a few individuals had found shelter in or around spring outflows flowing through the bottom muds in the large pool at Monsanto Well was supported by the discovery of a series of spring boils in 0.5 meter (diameter) depressions 0.1 meter deep in the rock bottom of the deepest part of the pool following scouring by a flood just prior to our August 1978 visit.