Ultrasonic tracking of mature cutthroat trout (Salmo clarki) from Little Thumb Creek displaced to the open water of Yellowstone Lake in 1969 suggested, but did not conclusively demonstrate, that trout from the west side of the lake have a westward compass mechanism, similar to the eastward compass mechanism of east side trout. Anosmic trout from Clear Creek made variable movements, but some exhibited the same patterns as control trout and horned. Experimental trout and control trout from Clear Creek retained prior to ultrasonic tracking under a 6 hr early photoperiod and a normal photoperiod, respectively, exhibited similar movements, most of which were eastward or southeastward as is typical of Clear Creek trout. Either they do not use a time-compensated sun compass mechanism, or we were unsuccessful in shifting the biological clock. Cloud cover apparently did not alter fish movement patterns.