Any person associated with the sardine industry is well acquainted with the progressive increase in length of the sardines taken throughout a fishing season. Small and mixed sizes are taken in the summer months, while the winter fishery consists chiefly of large fish. Such a seasonal variation is not common or, at least, has not been reported for any other important commercial fish in California. It is of primary interest, then, to define this trend in the sizes of sardines throughout each season and to compare the trends from year to year. During the past eight years, 1919–1927, the California State Fisheries Laboratory has made a detailed study of the sardine catch at Monterey. These records supply data for lengths of a representative sample of the catch for the entire fishing season and thus reflect changes in size from day to day, week to week and month to month.