AbstractThe article describes characteristics of the life cycle of potentially toxicogenic diatoms Pseudo-nitzschia calliantha Lundholm, Moestrup et Hasle and P. pungens (Grunow ex PT Cleve) Hasle living in the Black Sea. The rate of decrease in the length of cells when kept in culture has been studied. The duration of the life cycle and the critical size limit determined transition from one phase of the life cycle to another have been defined. The duration of the prereproductive phase was calculated. Changes in cell length are shown in P. calliantha from 147 µm in the largest initial cells at the beginning of the life cycle to 21 µm in extremely thinned ones, which made it possible to expand the previously known boundaries of the size range characteristic of the species. For P. calliantha, the absence of dependence of the size of the initial cells on the size of the parental cells is shown.