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and declined to 15,600 cfs by June 14. Water temperature taken at the point of collection for ripe females with strippable eggs ranged from 13.5 to 18 C in the Green River (average 16 C) and was 15 C in the Duchesne River. General habitat use of the razorback sucker was observed from capture locations and radiotelemetry. Very few razorbacks were captured during the nonbreeding season when they apparently select deeper waters in the shoreline and main channel runs and eddies. No razorbacks were captured in whitewater canyons of the Green Basin, and the fish evidently does not prefer these habitats. The best habitat information during nonbreeding season was obtained from one razorback sucker radiotracked from April to June 1980, in the Duchesne and Green rivers. It selected depths ranging from 0.6 to 2.4 m and velocities ranging from 0.1 to 0.5 mise Preferred substrate types were sand/silt in shoreline and main channel habitats. The fish selected midchannel and deep shoreline habitats in the Duchesne River. During flood stage it moved into the mixing zone of the Duchesne and Green rivers (eddy) and was last observed on a submerged sand bar (run) in the Green River in August. Data obtained from captured razorback sucker also indicated a preference of sand/silt substrates, usually in quiet water habitats. A water velocity less than 0.3 m/s was recorded for 80% of razorbacks captured. DISCUSSION Determinations of razorback sucker distribution and relative abundance have been encumbered because of the difficulty in capturing these fish on a year-round basis. This is illustrated by Vernal collections in the Green River from 1979-81, in which only five razorback were collected in a basin- wide systematic sampling program, but 92 were collected in early spring samples taken in the vicinity of spawning areas (then unknown) during the same calendar years. A concerted effort in 1984 yielded 102 razorbacks in a 1 1/2 month period. It is yet unknown whether these spawning aggregations occur in many localized areas throughout the basin, or are limited to only the few areas we have identified to date. The sizes of razorbacks collected in the Green River are much smaller than those from the upper mainstem Colorado River and from the lower Colorado River Basin. Minckley (1983) indicated that razorbacks in the lower Colorado River appeared to be old fish, and reported males as large as 640 rom and females up to 740 rom TL. McAda and Wydoski (1980) reported that razorbacks in the Upper Colorado River mainstem were also large fish, with males as large as 520 mm and females up to 620 mm TL. Valdez et al. (1982) found no difference between males and females with respect to size, but reported that razorbacks in the upper mainstem Colorado appeared old, and averaged 554 mm TL and reached sizes of 635 mm. Mature razorbacks collected in spawning condition from the Green River were smaller than those reported in other locations, as noted previously by McAda and Wydoski (1980), with males averaging 502 mm (largest 512 mm) and females averaging 541 rom (largest 586 rom). 200