The round whitefish, though rarely abundant, is widely distributed in northern waters. It is one of the least studied of the coregonines; the present report is but the second for Great Lakes waters. Commercial production in Lake Michigan has been tightly confined to the northern portion. The period 1924-30 showed the best production: 200,000 to 359,000 pounds. Since 1956, production has been around 10,000 pounds or less. The present age and growth study is based on 208 fish collected by gill net in December 1951. The relation between total length in inches (L) and the weight in ounces (W) is described by the equation log W = -2.7232 + 3.2940 log L. Age-group III made up 66.3 percent, and age-group IV, 20.6 percent of the sample; age-groups V, VI, and VII combined contributed only 5.9 percent. The average length for all fish in the sample was 14.5 inches. Growth was calculated from a previously published linear body-scale relation with an intercept of 1.1 inches on the axis of fish length. The increments of calculated length declined steadily from a maximum of 4.6 inches the first year to 1.0 inch the eighth. A 3-year-old Lake Michigan fish (12.3 inches) is as long as a 5-year-old Lake Superior fish, and an 8-year-old Lake Michigan fish (18.9 inches) is 0.9 inch longer than the oldest (12 years) from Lake Superior. The smallest mature males and females were in the length intervals 12.0-12.4 inches and 13.0-13.4 inches, respectively. All males over 1.4 inches and all females over 14.9 inches were mature. The youngest mature males were in age-group II; 36 percent of II-group males but none of the females were mature. All fish older than age-group III were mature.