The structure of the previtellogenic oocyte pool was studied in rainbow trout at the age of 10 and 20 months. It was shown that percentage of oocytes of different size groups in the females of the same age differs significantly and does not depend on length and mass of the fishes. The meiotic oocytes entered previtellogenesis in multiple cycles that was followed by the structural change of the whole their pool. The moments of transition from the fund of meiotic oocytes to the fund of previtellogenic oocytes, as well as from individual size groups of previtellogenic oocytes into subsequent size groups are not determined by the size of the fish, but are related to the dominance phase of the oocytes of one or another size group. With these multiple “waves” in the structure of the oocyte stock, a general increase in their number is associated. As a result the percentage of oocytes in different size groups changed. At the age of 10-20 months the number of previtellogenic oocytes in rainbow trout females increased 9 times, at average from 25,8 to 239,5 thousand.