The ecology of mountain whitefish (Prosopium williamsoni) in the upper Sheep River was studied during 1972 and 1973. Mountain whitefish spawned from late September to mid-October in shallow, fast, midstream areas. The number of spawning fish on each spawning site was small (two to 20 fish per site). Absolute fecundity (eggs/fish) increased as the females became older; however, relative fecundity (eggs/g of fish) remained comparatively stable for all age classes. The linear regression of absolute fecundity (Y) on fork length (X, in mm) is described by the formula: Y = 4347.6 + 50.7 (X – 301.5). Growth is rapid except in the older age classes of 7 and 8. The weight (Y, in g) length (X) relationship (sexes combined) is described by the formula: log Y = 2.1967 + 3.2108 (log X – 2.3710). The relationship of body (Y) to scale (X) growth is described by: log Y = 2.0891 + 0.9542 (log X – 2.4367). Recruitment into the population of the upper Sheep River appeared to be at 3 years of age with an annual natural mortality rate of approximately 33% on 3-year-old and older mountain whitefish. The diets of mountain whitefish consisted primarily of dipterans, ephemeropterans, trichopterans, and terrestrial invertebrates.