The growth and age structure of mountain whitefish, Prosopium willlamsoni, was compared from three sample locations in the North Fork Clearwater River, and a tagging study initiated to assess movement and migratory behavior. No difference was detected in growth or age structure of whitefish in two streams of the upper drainage, and juveniles representing age groups I and II were generally absent from upper river populations. Age groups I and II whitefish from the lower North Fork Clearwater had significantly greater growth than similar fish from the upper river. The sample from the lower river contained whitefish as old as age IV, whereas fish to age XIV were found in the upper reaches.