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(Jensen in Rinne et al., in press). To this end, detailed operating procedures are followed to preserve genetic integrity and health of all fishes. Each species is maintained in a separate pond to prevent loss due to hybridization and predation by or competition with other species. Congeneric species are maintained in widely separated ponds. Seines, dipnets, and personal gear are dried, disinfected, or changed when work proceeds from one pond to another. Taxonomic identification of incoming species is verified and they are also examined for presence of parasites or disease organisms that could endanger other captive-held fishes or impact reintroduction efforts. HOLDING To date, objectives have been met with varying degrees of success (Jensen in Rinne et al., in press). Perhaps the most outstanding fact is that never before have so many species of imperiled fishes from a wide diversity of native habitats been formally held and cultured at a single location. Comanche Springs, Leon Springs, and desert pupfish, Yaqui chub, and beautiful shiner have been readily maintained. Pupfish populations are typically cyclic, with numbers peaking in August and September and declining during the winter. Chub and shiner populations are usually more stable although their numbers follow the same pattern. All are multiple spawners with young fish present throughout the reproductive season, usually mid-April through early October at Dexter. Two live-bearers, Big Bend gambusia and Gila topminno w, have produced similarly large populations at Dexter but experience extreme annual population fluctuations; it is not unusual to experience a 90X loss during the winter. To ensure survival, ene hundred or more individuals are overwintered in aquaria and restocked outdoors in the spring. The larger ,species overwinter successfully at Dexter. Chihuahua chub was thought to be extinct for nearly 40 years until it was "rediscovered" in 1975. Following extensive flooding during December 1978, only LO fish could be captured in the wile; they were brought to Dexter in April 1979. ;Chihuahua chubs- have numbered more than 1,000 and reintroduction of hatchery-reared fish is planned during 1986. Some fish species are held at Dexter NFH to safeguard their genotypes while field investigations are underway to determine their status. The Dexter facility is not intended to be a final holding place for imperiled fishes, and if the status of natural -35-