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Introduction The Owens tui chub (Gila bicolor snyderi) is very similar in form, ecology, and behavior to other subspecies of tui chub inhabiting many Great Basin drainages. This subspecies is found in the Owens River drainage in east-central California, a subsystem of the Death Valley hydrographic basin (Fig. 1). During pluvial periods of the Pleistocene this drainage was continuous with the Death Valley system and other systems to the north and south, and hence shows many close ichthyofaunal relationships to these presently isolated drainage basins (Miller 1946; Hubbs and Miller 1948). The present distribution of this fish is the result of the invasion and subsequent isolation of the Lahontan form (G. b. obesa) during the Pleistocene (Miller 1946). The Owens tui chub shows close taxonomic affinities to this subspecies (Miller 1946). The Owens tui chub is both state and federally listed as endangered and is presently restricted to five locations: the Owens River immediately downstream from Crowley Lake; the Owens Valley Native Fish Sanctuary; Little Hot Creek; springs and ditches along the west shore of Owens Lake; and the Hot Creek headsprings, where this study took place (Fig. 1). Factors contributing to their endangered status are predation by exotic species, water development, and, most importantly, hybridization with the Lahontan tui chub, which was introduced into Crowley Lake and has rapidly spread throughout the lower reaches of the drainage. Only those populations that are isolated by barriers have escaped introgression. The Hot Creek headsprings are located in the Long Valley Caldera which erupted approximately 700,000 years ago, forming the Bishop Tuff and the present geomorphology of Long Valley (Bailey et al. 1976) (Fig. 1). The two headsprings which comprise the habitat for this population, known as the CD and AB headsprings, are part of a complex of springs which emanate from the edge of a basalt flow and coalesce to form Hot Creek, which flows into the Owens River just above Crowley Lake. These two springs and three others supply water to the Hot Creek State Fish Hatchery (Fig. 2). The CD headspring discharges into a small reservoir then flows approximately 200 m to its terminus. It flows at about 0.35 m Is, which varies only slightly throughout the year, and has a constant temperature of about 14.5 C. It ranges in depth from 0.1 m to 0.7 m. The AB headspring is approximately 130 m in length, has a flow about the same as the CD headspring, and has a fairly constant temperature of about 15.0 C. Depth ranges from 0.1 m to 0.75 m. Populations of rainbow trout (Oncorhynchus mykiss) and cutthroat trout (0. clarki) coexist with the chubs in the headsprings and no other fish species are present. Both headsprings are designated critical habitat and they both support a lush growth of emergent and submergent aquatic plants, which include: water-cress (Nasturtium officinale), water fern (Azolla 12