Dynamic of the reproduction of the trematodes Notocotylus imbricatus (Notocotylidae), Echinostoma caproni (Echinostomatidae), Sphaeridiotrema globulus and Psilotrema tuberculata (Psilostomatidae) parthenites was observed. The formation of generative cells takes place only as a result of undifferentiated cells proliferation and following differentiation. These processes and early stages of embryogenesis are taking place only in the special reproductive organ--germinal mass. The germinal mass is always formed at the posterior part of the body. The process of reproduction had been finishing to the beginning of the generating of cercariaea of new age by parthenites. Rediae of different generations in fact stop producing new generative cells with the beginning of the appearance of a new generation.