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species in general (Funk 1957) and in other migratory cyprinids (Stott et al. 1962). The Colorado squawfish apparently changes from sedentary to mobile behavior with sexual maturity. Seethaler (1978) reported that 76% of male and 50% of female Colorado squawfish were mature by 503 mm TL. Table 4 indicates the average size of four sedentary Colorado squawfish was 476.5 mm and the average size of 11 mobile Colorado squawfish was 565.3 mm. Thus, the apparent difference in behavior could be explained by differences in sexual maturity, a hypothesis supported by the dissection of two Colorado squawfish reported from the Green River in 1980 (Tyus et al. 1981). The documentation of a Colorado squawfish spawning migration in the lower Yampa River in 1981 provides an explanation for the behavior of other radiotelemetered fish in the Green and White rivers.- The long distance movement of two Colorado squawfish (No's 028 and 165) implanted in the Green River in 1980 and one implanted in the White River in 1981 (No. 1624) was probably associated with spawning behavior, since the movement occurred during the spawning season. The movement of two Colorado squawfish (No. 028 and 1624) into Gray Canyon in 1980 and 1981 may indicate the existence of a second spawning ground. Radiotagged fish entering the lower Yampa Canyon in 1981 apparently selected a few restricted and specific habitats for spawning. As in- dicated in Figure 6, all fish returned to their former upstream or downstream locations after spawning. This suggests a homing ability for which the mechanism is unknown. Local Movement 04, The results of diel studies of Colorado squawfish in 1980-81 are inconclusive. Observations of Colorado squawfish in 1980, indicated they were more active in the Green River between the hours of 0800-1600. But these results were not supported by further work in the Green, White and Yampa rivers in 1981 (Fig. 9). Adult Colorado squawfish appear to be very active and move at any hour. Diel study results for all rivers were partitioned into prespawning (before June 15), spawning (June 15- July 15) and postspawning (after July 15). These data do not indicate a strong diel rhythm until the postspawning period when movement activity was greatest from 0800-1600 hours (Fig. 10). During the spawning period more activity in the Yampa River occurred at night. This nocturnal movement could be associated with the spawning movements, since Manteifel et al. (1978) reported twilight migration in cyprinids in Russia. Muller (1978) found diel activity in several fish species and concluded. that light was a more important influence for this activity than water temperature. The activity level of Colorado squawfish was evaluated for each week by computing the average distance moved in 15 minutes during diel studies (Fig. 11). Although considerable variation existed between fish, the activity level was generally high throughout the year with the highest activity observed in weeks 25-29, the proposed late June, early July, spawning period. 18