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ABSTRACT A laboratory study was conducted to determine the effects of four constant temperatures (18, 22, 26, and 3 0°C) and three fluctuating temperatures (18, 22, and 26°C; diel fluctuation ~ 2.5°C) on early development and survival of Colorado squawfish Ptychocheilus lucius, which is listed as an endangered species by the U. S. Department of Interior. Average hatch in constant and fluctuating temperatures was 72% at 18°C, 67% at 22°C, 62% at 26°C and 38% (constant temperature only) at 30°C. There was no significant difference in hatch between constant and fluctuating temperatures.. Average survival of larvae to 7 d in constant and fluctuating temperatures was 68% at 18°C, 64% at 22°C, 83% at 26°C, and 13% (constant temperature only) at 30°C. Survival of larvae at 30°C may be confounded by relatively low hatch of embryos and poor condition of larvae. Survival of larvae was 10-31 % higher in fluctuating than in constant temperatures. Incidence of abnormalities was 2-22% at 18-26°C and 100% at 30°C. Differences in abnormality rates were not detectable between constant and fluctuating temperatures. Time to initiation of hatch, swim bladder inflation, and exogenous feeding was shorter at higher temperatures. First feeding occurred about 31 h earlier in fluctuating temperatures than in constant temperatures. Differences in lengths of larvae at hatching (d 1) and at d 7 at the various test temperatures were small and not considered ecologically significant. Tolerance of a relatively wide range of warm water temperatures by Colorado squawfish embryos and larvae may reflect the historically variable Colorado River environments in which the species evolved. Cold summer water temperatures caused by mainstream dams have eliminated Colorado squawfish from portions of its historic Colorado River basin range. Water temperatures that more closely reflect historic 7