On the basis of age-composition of fish taken by commercial nets in 1949 and 1950, the whitefish in Big Bay de Noc, in the upper Michigan peninsula, appear to be a population separate from that of northern Lake Michigan. There was a change in the age-structure of the commercial catch of whitefish in Big Bay de Noc during the period 1949 to 1954. In 1949 and 1950, several year-classes made important contributions to the catch, but after 1950, Age-III fish provided 85 to 90 percent of the annual catch. Possible reasons for this change in age-structure of the catch are discussed. With the exception of two localities in northern Lake Michigan, growth rate of whitefish in Big Bay de Noc and in the lake was similar. Annual differences in growth in Big Bay de Noc were noted. Fish of the larger year-class of 1949 showed greater first-year's growth than other year-classes, suggesting that the factors responsible for good growth may also favor high survival. Whitefish of older age-groups also showed indications of greater growth during 1949 than did fish of the same age in other years.