Fall and winter movements of and habitat use by cutthroat troutOncorhynchus clarkiwere studied with radiotelemetry in low, mid, and high altitudes of a river system to evaluate the timing and extent of habitat shifts when fish moved from summer feeding areas to overwintering areas. The movements of 20 fish were monitored from August to November 1991, and 17 fish were monitored from October to December 1992. Cutthroat trout moved out of summer habitats in mid-September, and many made a two-stage shift in habitat use from summer to winter that was associated with anchor ice formation. When cutthroat trout were excluded from fall habitats by anchor ice, they moved to overwintering areas less likely to be influenced by frazil and anchor ice: deep pools with ice cover or areas where water temperatures were higher than the rest of the stream because of springs or upwelling warm groundwater. Cutthroat trout used deeper water and smaller diameter substrates in winter than in summer but used less cover during winter. Because of decreases in discharge and exclusion by anchor ice, cutthroat trout were forced into restricted suitable habitat, which resulted in large aggregations of fish. Cutthroat trout moved 0.0–7.6 km from summer to overwintering areas. They were stationary most of the time; when they did move, most movements exceeded 100 m. The number of movements made decreased from summer to winter.