Little is known about the periodicity of mortality of brook trout (Salvelinus fontinalis) between the egg stage at midwinter and the fingerling stage the following fall. In a 300-yard (0.77-acre) section of the Pigeon River, Otsego County, Michigan, the average instantaneous mortality rate of young brook trout was computed to be 0.02419 per day from emergence in March to June 26, 0.00691 per day from June 26 to July 24, and 0.00519 per day from July 24 to September 19, 1961.