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fish at the conclusion of each experimental trial. Mean length of one-week old Colorado pikeminnow larvae were 8.7mm .(range 8.5-8.9mm, n=30), the three-week old larvae were 9.8mm (range 9.5-10.1mm, n=34), and the six-week old larvae were 12.1 mm (range 11.2-13.8mm, n= 47). Mean length of the two-week old razorback sucker larvae was 10.3 mm (range 10.1-10.4mm, n=32). We captured a limited number of juvenile Colorado pikeminnow (n=60) from backwaters of the Green River in September and November 1987, and these were kept in 110 1 aquaria. All of the juvenile Colorado pikeminnow (36-week old) in the study were measured, and the average length was 52.5 mm (range 32-82mm). Daylight conditions were provided by five incandescent light bulbs (white, 25W) in a linear array over the test tank. The lights were on a cycle of 12 h light and 12 h dark with dawn and dusk occurring instantaneously at 0700 and 1900. One 25W red light provided a continuous, diffuse illumination that aided observations at night. A small flashlight was used to aid in locating larvae at the end of the experimental trials. The influence of three different flow rates on movements of four age classes of young Colorado pikeminnow and one of razorback sucker was evaluated in darkness and in light. Subjects from each age group were tested at each of three flow rates during light (0700-1900h) and dark (1900-0700h) conditions. Each combination of age, light, and flow rate was replicated five times with five fish in each replicate. No larval fish was used for more than one trial. Individuals from the group of 36-week old pikeminnow were used for two trials each: one with no flow and one with flow (either the slow or moderate rate). For these fish, each combination of flow rate and light level was replicated three times. Before fish were introduced in a trial, the middle chamber of the tank was isolated from the others by sliding metal gates over the ports. Fish were placed in the middle chamber and allowed to acclimate for 15 minutes with no flow. Ports were then opened and the trial initiated. The number of larval fish in each chamber was recorded at 5, 15, 30, and 60 min. For the larger, and more active, 36-week old fish, observations were recorded every three minutes until the trial was terminated at 60 minutes. Most of the comparisons deal with movements recorded at the end of 15 min, because this minimizes the opportunity for fish of each age group to reach the end of the tank, explore it, and move back in a different direction. Timed observations of the distribution of fish among the seven chambers provided a basis for calculating two measures of movement: TA (total activity) and DA (directed activity), which are defined below. Both are measures of net movement (i.e., movement to a final position and not the path by which that position was reached). In this paper, net movement is used as a reasonable surrogate for total movement. TA= ~ n~ 528