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INTRODUCTION Understanding the life history of the endangered Colorado squawfish (Ptychocheilus lucius) is prerequisite to developing strategies for its recovery. Endemic to the Colorado River basin, this species is noted as the largest cyprinid native to North America.. The historic range of this piscivore has been reduced by 801/o (Tyus 1990). Extant populations occur only in the upper Colorado River basin (i.e., upstream of Glen Canyon Dam) in warm-water reaches of the Colorado, Green and San Juan river systems. The species has integrated movements into its life strategies, including annual, potamodromous, spawning migrations (e.g. Tyus and McAda 1984, Tyus 1985, Tyus 1990, McAda and Kaeding 1991), and the subsequent downstream drift of emerging larvae (e.g. Haynes et al. 1984, Nesler et al. 1988, Tyus 1990, Tyus and Haines 1991). Upon emergence, Colorado squawfish larvae drift downstream and are soon widely distributed. By late summer they can be collected with seines from backwaters and other low- velocity habitats where they largely remain through the postlarval phase (Haynes et al. 1984, Tyus and Haines 1991, McAda and Kaeding 1991). By fall, many young-of-the-year (YOY) in the Colorado River become concentrated in the lower 103 km (above the Green River confluence) where gradient is low and sand-bar-formed backwaters are most plentiful (Valdez et al. 1982, McAda et al. 1994). There they rear through the first year of life. After attaining about 100 mm in length, Colorado squawfish are infrequently collected; however, once > 250 mm long, they become more susceptible to capture with trammel nets and electrofishing. Size of Colorado squawfish in the Colorado River varies longitudinally: early life stages are found primarily in the lower reaches and adults are found primarily in more upstream reaches (Valdez et al. 1982, Osmundson and Burnham 1996). This differential distribution of life stages was also noted in the Green River system by Tyus (1986) and Tyus et al. (1987). Valdez et al. (1982) suggested that: "... YOY move downstream with age, and juveniles and adults subsequently move upstream with increasing age." Tyus (1986), in reference to the Green River population, similarly noted that : "A net long-term movement of juveniles must occur to populate adult areas upstream, probably in the late young-adult stage,..." These distributional patterns of various life stages of Colorado squawfish indicate that population dynamics occur at the landscape scale and that large sections of river are needed to meet the needs of all life stages in a given population. Evidently, the best spawning habitat occurs in middle to upper reaches. Long-distance drift provides a mechanism for delivering emerging larvae from these areas to more optimum nursery habitat, generally located far downstream (Tyus 1990). Though hypothesized, patterns of river-wide dispersal of adults and subadults from downstream areas where they reared to upstream reaches has not been documented and is poorly understood. Our primary objectives were to describe the distribution of subadult and adult Colorado squawfish in the Colorado River and document their dispersal patterns. We