Although the whitefish has by no means ranked first from the standpoint of production, it has always been an important commercial species in Lake Erie. Trends in the output of whitefish have differed in the United States and Canadian waters of the lake. The 1893–1946 average annual yield of 1,201,000 pounds in the United States was only 38.3 percent of the 1879–1890 mean of 3,133,000 pounds, whereas in Canada the more recent (1907–1946) average annual take of 1,397,000 pounds has been 5.48 times the 1871–1906 mean of 255,000 pounds. The United States fishery was centered in the western part of Lake Erie (61.5 percent of the production in Michigan and Ohio) before 1921 and in the eastern part (62.6 percent in Pennsylvania and New York) in 1921–1946. The eastern part of Lake Erie (east of Port Burwell) dominated the Canadian production in 1900–1909 (65.4 percent) and in 1922–1946 (57.2 percent) but the western end was the more productive in 1871–1899 (79.8 percent) and 1910–1921 (69.7 percent). Ages were determined and individual growth histories calculated from the examination and measurement of the scales of 3,399 Lake Erie whitefish captured off four ports (Sandusky, Lorain, and Conneaut, Ohio, and Erie, Pennsylvania) over the period, 1927–1930. The number of specimens used for the investigation of other phases of the life history varied according to the amount of data available or required. Age-group III was typically (but not invariably) dominant in random samples from gear employed for the commercial production of whitefish (trap nets, pound nets, and large-mesh gill nets). The same age group also dominated most samples of the marketable catch (that is, whitefish that equalled or exceeded the minimum legal weight of 1 3/4 pounds) taken in late summer, autumn, and early winter. Age-group IV, however, was strongest among marketable fish from trap nets in early July although the III group was dominant in the random samples from the same nets. Apparently the members of a year class normally dominate the commercial catch about one year but this year extends over parts of two years of life (latter part of the fourth and early part of the fifth). The oldest whitefish in the collections were in the seventeenth year (age-group XVI). The year classes of 1922 and 1926 were much stronger than average whereas the 1923 year class seems to have been exceptionally weak. No correlation was detected between limnological-meteorological conditions and the strength of the year classes. Whitefish collected off different ports exhibited differences of growth rate that were at times rather large. The distorting effects of such factors as selection on the basis of maturity, annual fluctuations in growth rate (in combination with differences in the year of capture), and gear selection were held to be sufficiently great, however, to render doubtful the real biological significance of the observed variations in growth. Consequently the data for all samples were combined to obtain general growth curves. Female whitefish averaged longer and heavier than male fish of corresponding age. The advantage of the females with respect to calculated lengths tended to increase during the first three years of life and thereafter remained nearly constant at about one-half inch total length (10 millimeters of standard length). The advantages of the females with respect to weight increased consistently from 0.01 pound at the end of the first year to 0.36 pound at the end of the eighth, dropped to 0.32 pound in the ninth year, and increased again to a maximum of 0.47 pound at the end of 12 years. The maximum growth in length (sexes combined) occurred in the first year of life (calculated growth of 6.9 inches, total length). From this value the calculated annual increments declined rapidly to 0.7 inch in the seventh year. The later increments varied irregularly, ranging from 0.7 inch in the eighth and ninth years down to only 0.3 inch in the fifteenth and sixteenth years. The Lake Erie whitefish was a foot long in a little less than 2 years, 18 inches in about 4 years, and 2 feet in slightly under 12 years. At the end of 16 years the calculated length was 25.6 inches. The calculated annual increments of growth in weight increased from 0.10 pound in the first year to a maximum of 0.76 pound in the third. In the succeeding years the increment decreased consistently to 0.33 pound in the twelfth year. The values in the thirteenth to sixteenth years varied irregularly, ranging from 0.22 to 0.34 pound. The minimum legal weight of 1 3/4 pounds was attained toward the middle of the fourth growing season. The Lake Erie whitefish reached the weight of 4 pounds in between 7 and 8 years, and of 6 pounds in about 13 years. At the end of 16 years the calculated weight was 6.87 pounds. Analyses of the annual increments of length revealed that the growth of whitefish captured from the spawning run off Sandusky and Lorain rose from 3.2 percent above the 1924–1930 mean in 1924 to a peak of 15.0 percent above average in 1927 and then declined to a minimum of 25.0 percent below average in 1930. There is evidence that these annual fluctuations in growth rate were correlated negatively with fluctuations in the turbidity of the water off Erie, Pennsylvania (to our best knowledge the whitefish spends the summer months in the eastern part of the lake), in certain months (especially May and June) and/or correlated positively with the amount of rainfall in July and August at the same locality. Comparisons with data on the growth of the Lake Huron whitefish revealed that Lake Erie fish were the longer during the first 5 years of life and the shorter at the end of the sixth and later years. The Lake Huron whitefish did not, however, gain the advantage in weight until the seventh year. Whitefish grew much more slowly in both length and weight in Lake Ontario than in either Lake Huron or Lake Erie. The weight of the Lake Erie whitefish increased to the 3.1523 power of the length. Agreement between empirical weights and those computed from the length-weight equation was reasonably good at lengths represented by fair numbers of fish. The total length corresponding to the minimum legal weight of 1 3/4 pounds was calculated as 16.9 inches. The rather limited data on the monthly fluctuations in condition indicated that the coefficient K of immature fish declined continuously from August to December. A similar though less pronounced decline of K of mature fish occurred from August to October. At spawning in November and December, female whitefish lost an additional 11 percent of their body weight. No loss of weight at spawning could be demonstrated for the males. The available records indicated the relative abundance of the sexes to be approximately equal in samples collected in the summer and early autumn. Males were strongly predominant (78.6 percent), however, in spawning-run samples. In these collections the percentage of males decreased markedly with increase in age. No trend could be detected in the variation of the sex ratio within the spawning season over the period of time (nearly 4 weeks) for which there were records. Although exceptional individuals of either sex may mature at the end of 2 years of life (age-group I) male whitefish do not mature in appreciable numbers until the end of the third year (age-group II) or females until the end of the fourth (age-group III). Apparently most or all males are mature as age-group III, but there is evidence that considerable numbers of females (possibly a majority) are first mature as members of the IV group (end of fifth year of life). Whether Lake Erie whitefish are ever immature as the V group or older is not known. Spawning commenced during the second week of November and was continuing actively at the time of collection of the last samples at the end of the first week of December