Ages were determined and individual growth histories were calculated from the examination and measurement of the scales of 1,649 kiyis captured at seven localities in Lake Michigan in 1931 and 1932. The numbers of individuals employed for the investigation of other phases of the life history (such as length-frequency distributions, length-weight relationship, and sex ratio) varied according to the amount of data available or required. Age-group IV was dominant in the 1931 collections from Racine, Port Washington, and Kewaunee, Wisconsin, and age-group V dominated the 1932 samples from the Fox Islands and from three localities southward of Manistique, Michigan. A trend was noticeable toward an increase in average age from south to north. Among the explanations suggested for the observed differences in age composition were: Variation with latitude in the natural span of life; differences in fishing intensity; fluctuations in the strength of year classes (to account possibly for the shift in the dominant age group from 1931 to 1932). The oldest male kiyi belonged to the VII group and the oldest female was a member of the X group. The possible distorting effects of such factors as gear selection traceable to differences in the mesh sizes of nets fished in 1931 and 1932, selection by nets on the basis of the condition (K) of the fish, and local variations in fishing intensity and hence in the selective destruction of rapidly growing individuals in the fishery were held to be sufficiently great to render doubtful the significance of most of the observed local differences in growth rate. Kiyis from all samples were combined to determine the general growth in length. The growth in weight of the Fox Islands fish, however, was considered separately as these fish were consistently lighter than kiyis of corresponding length from other localities. The Lake Michigan kiyi grows slowly, with the females growing slightly more rapidly than the males. The grand average calculated lengths indicated, for example, that the females did not attain a total length of 10 inches until the fifth year of life or the males until the sixth. Similarly, the calculated weight of 4 ounces was not reached until the fifth or sixth year (with the actual time varying with sex and locality). The season's growth of the kiyi probably begins sometime in May and most or all of the growth is completed by the end of August. The calculated lengths of the age groups exhibited large discrepancies that differed from “Lee's phenomenon” as ordinarily observed in that the data for the later rather than the earlier years of life were affected most severely. Chief among the factors held responsible for these discrepancies were gear selection and the selective destruction of the more rapidly growing individuals in the fishery. Errors inherent in the (direct-proportion) method of computing growth from scale measurements were considered to have been unimportant. The Lake Michigan kiyi exhibits growth compensation–the tendency for the smaller of the young fish to have the more rapid growth in the later years of life. Comparisons with the average lengths and weights of the age groups of the Lake Ontario kiyi given by Pritchard (1931) indicated the Lake Michigan fish to be the larger at the earlier ages (age-groups II and III) and the smaller at the later ages (age-groups IV to VI). The length-frequency distributions of the age groups exhibited extensive overlap. As many as eight age groups were represented in a single centimeter interval of length. The length frequencies and average lengths of all fish collected, arranged according to the mesh sizes of the gill nets by which they were captured, revealed that the selective action of these nets in the taking of kiyis was much more obvious in the numbers of fish in the catch than in their average size. As an illustration, in 1930–1931, the 2 3/4-inch mesh nets took fish that were only 0.1 inch longer than those in 2 1/2-inch meshes but captured less than one fourth as many. Gill nets fished in northern Lake Michigan in 1932 captured kiyis that averaged 0.2 to 0.4 inch longer than those taken in the same meshes in southern Lake Michigan in 1930–1931. Because of the more slender form of kiyis from the northeastern island region of Lake Michigan, data on the general length weight relationship were compiled separately for fish of that area and for those of the great central basins of the lake. In both regions the weight increased to a power slightly greater than the cube of the length. Available information on condition indicated that the coefficient (K) was higher in August and early September than in May, June, and July. Condition declined from early September to October and early November–the latter period the time of most active spawning. Spawning itself was accompanied by an additional loss of about 12 per cent of the body weight of females and of somewhat less than 2 per cent of the weight of males. Analysis of the variations of K within a group that was homogeneous with respect to age, sex, maturity, and time of collection revealed that a net of a particular mesh size tends to take the heavier of the shorter fish and the lighter of the longer fish within its range of effectiveness. Among fish of the same length the values of K tended to increase with increase in the mesh size of the nets employed for their capture. Practically all fish in the samples were mature (only 11 immature in more than 6,000). These “immature” fish were probably “non-functional” since all of them approached or exceeded the average length of the mature kiyis. Females were strongly predominant in the collections at all seasons but were relatively more plentiful during the summer (90 per cent of the total) than during the spawning period (75 per cent). Possible factors contributing to this predominance of females and to the change in the sex ratio at the spawning season were discussed. A decrease in the relative abundance of males with increase in age appears to be characteristic of the kiyi. This decrease indicates a differential mortality of th sexes (greater relative destruction of males in the spawning period when they are unusually abundant or a greater natural mortality rate for the males). Current fishery regulations on mesh size and closed seasons afford the kiyi good protection but offer no guarantee against depletion from too intensive fishing.