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EXECUTIVE SUMMARY Growth and survival rates are fundamental components of most demographic studies of long- lived species and are essential to understanding population dynamics. Determination of population status and trend and identification of factors limiting population increase are requisite to formulating recovery goals and management strategies for endangered species. The primary objective of this study was to refine previous estimates of Colorado squawfish growth rate in the Colorado River by minimizing the use of scale analyses, a technique deemed unreliable for estimating growth in older fish. A secondary objective was to estimate adult survival rate for this population. A third objective was to estimate age of Colorado squawfish of various sizes and provide some estimate of potential longevity. Growth and adult survival rates of Colorado squawfish (Ptychocheilus lucius) in the upper Colorado River were estimated using fish captured from 1990 through 1995. Mean annual growth rates of 3- to-6-year-old fish were estimated from measured lengths of individuals aged by scale analysis, and ranged from 32.2 (6-year-olds) to 82.0 (3-year-olds) mm/year with coefficient of variation near 1.0. Growth rates for older fish were estimated from changes in total length (TL) of marked and later recaptured individuals. These annual growth increments were averaged for fish within 50 mm size groups. Rates were highest for fish 400- 449 mm TL with length increases averaging 42.7 mm/year, and declined to 22.2 mm/year for fish 500-549 mm TL. For fish -> 550 mm TL, growth increments averaged 8.7 mm/year, but varied (although not significantly) from 3.9 to 15.0 mm/year depending on the size group. Length frequencies by age for 8-year-old and older fish were estimated by simulating growth beginning with the distribution of lengths for 7-year-old fish and adding annual growth increments. A simulated population of 10,000 fish was used with growth increments randomly selected from the measured distributions of growth increments within 50-mm size classes to provide an estimate of range in length at each age for 95 % of the population. Lengths of individuals varied by 100 mm for 10-year-old fish and by as much as 200 mm for fish 20 or more years old. The range of ages for a given length was also highly variable with ages differing by up to 20 years for fish > 800 mm M. Survival rates for fish Z 550 mm TL were estimated by comparing measured size distributions with simulated stable age and size distributions; these ranged from 0.83-0.88 (P < 0.05) with the best fit at 0.86. Also, in the Discussion, we offer an explanation on the current absence of very large fish often reported in historical accounts and discuss additional mortality factors arising during the past 75-100 years that might have lowered adult survival rates. Of these, choking on the spines of ingested channel catfish (Ictaluras punctatus), an introduced species, may be the most serious. Though lack of historical data on growth and survival rates precludes comparisons with past rates, estimates provided here can serve as a baseline for future population monitoring efforts. A-v