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INTRODUCTION Colorado pikeminnow Ptychocheilus lucius were historically distributed throughout warm-water reaches of the Colorado River basin from Wyoming, Utah, and Colorado south to the Gulf of California (Miller 1961). By the 1970s they were extirpated from the Colorado River basin below Glen Canyon Dam (entire lower basin) and sections of the upper basin as a result of major alterations to the riverine environment (Moyle 1976). Having lost some 80% of its former range, the Colorado pikeminnow was listed as endangered by the U. S. Fish and Wildlife Service (USFWS) in 1967 (Federal Register 32[43]:4001) and was later given protection under the Endangered Species Act (Federal Register39[3]:1175). Colorado pikeminnow in the upper Colorado River basin presently inhabit warm-water reaches of the Colorado, Green, and San Juan rivers and associated tributaries. The Green River and its two largest tributaries (White and Yampa rivers) support the largest and perhaps most viable (Gilpin 1993) population, while the San Juan River contains the smallest population (Platania et al. 1991). A third population persists in the upper Colorado River, but relatively low catch rates of adults and young during the 1980s (Valdez et al. 1982; Osmundson and Kaeding 1989; McAda et al. 1994) led U. S. Fish and Wildlife Service to conduct an in-depth investigation into the status and trend of this population during 1991- 1994. Although Colorado pikeminnow continue to reproduce and new adults are recruited to the population, strong year classes are infrequent and low river-wide abundance of adults limits the long-term viability of the population (Osmundson and Burnham 1998). After a three-year hiatus following the 1991-1994 sampling, another study was initiated to assess recent population status and assess trends. Annual, systematic, sampling of Colorado pikeminnow (> 250 mm long) was conducted throughout their range in the Colorado River mainstem during 1998-2000. The primary goals were to assess population trends by providing new estimates of population size and to assess recent recruitment activity by examining changes in population length frequency. Body condition of fish was also examined as a means to detect trends in food availability. Another goal, not addressed in the earlier study, was to determine the sex ratio of the population. Such information could help refine growth rate and longevity estimates and help refine estimates of the census adult population size needed to provide an effective population size (Ne) of 500, generally believed