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fish were allowed to ripen naturally in ponds until 29 May. Colorado squawfish were artificially spawned following injections with common carp Cyprinus c io pituitary (males) and human chorionic gonadotropin (females; Hamman 1981). The resulting embryos were shipped to Fort Collins, Colorado, in water at 18-21 °C within 24 h. Four groups of 30 viable embryos each were randomly assigned to each experimental treatment, placed in incubation chambers, and acclimated to test temperatures at about 2°C/h. Embryos were incubated in chambers constructed from 76-mm inside-diameter polyvinyl chloride (PVC) pipe with X60-um nylon screen attached inside and halfway down the 100 mm long pipe section with silicone caulk. Chambers were placed in a bath and filled with water delivered via a fine bore pipette. Holes were drilled at the base- of the chamber wall to allow water to flow out. Embryos were incubated at nominal constant temperatures of 18, 22, 26, and 30°C and nominal fluctuating temperatures of 18, 22, and 26°C (diel fluctuation of + 2.5°C). One group of embryos was incubated in each of four test chambers at each of the seven constant and fluctuating temperature treatments. Additional embryos and larvae were held in a fifth chamber at each treatment so that observations of development and fish length could be made without disturbing fish in the other four incubation chambers. Diel fluctuating temperatures were tested because diel temperature fluctuations of 5°C or more are common in the Yampa and Green rivers, Colorado and Utah, during the spawning season of Colorado squawfish (late June to early August, Nesler et al. 1988; personal observation KRB). Our fluctuating temperature cycle mimicked the natural cycle as follows: lowest temperatures occurred from 0100 to 0700 h (e.g., 19.5°C for the 22°C fluctuating treatment); warming occurred at a constant rate from 0700 to 1300 5