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include predation, competition, and starvation, which often interact with environmental conditions that influence growth to regulate recruitment of larvae (May 1974, Hunter 1981, Houde 1987, Bailey and Houde 1989). Most populations of endangered Colorado squawfish Ptychocheilus lucius of the Colorado River basin may be recruitment-limited (Tyus 1991). In Green River, which is the largest remaining population, annual densities of juveniles in fall (recruits) vary from near zero up to 75 fish/100 m2 of backwater habitat (Tyus and Haines 1991). However, the relative effects of discharge regime, habitat alterations, and introduced fishes on recruitment among and within years is poorly understood. I designed laboratory experiments (chapters 1-3) and a field study (chapter 4) to evaluate the relative importance of physical and biological factors on recruitment of age-0 Colorado squawfish. Experiments described in chapter one (Bestgen and Williams 1994) were used to evaluate effects of different water temperatures, and constant and fluctuating temperature regimes, on hatching success of Colorado squawfish embryos and survival of larvae to 7 d. In chapter two (Bestgen 1996), growth and survival of larvae was measured at four different water temperatures levels, constant and fluctuating temperature regimes, and at five different levels of food abundance. In addition, an experiment to determine starvation resistance of squawfish larvae was also conducted. Chapter three (Bestgen and Bundy; submitted to Transactions of the American Fisheries Society) details experiments to validate the use of otoliths in determining age and growth rate of Colorado squawfish larvae and juveniles reared at different water temperatures with constant and fluctuating regimes, and different food conditions. 2