The ultrastructure of reproductive organs of trematode parthenitae during the cold season was investigated for the first time. We used as material the rediae of a monoxenous hemiuroid digenean Bunocotyle progenetica parasitizing intertidal mud snails Peringia ulvae at the White Sea. Infected molluscs were collected in hydrological winter (March) from under the ice at a temperature of near-sediment water of –1.5–1 °C. The ultrastructural study of the germinal masses and the brood cavity of “winter” rediae revealed substantial differences from their “summer” state described in Podvyaznaya et al. (2019). The mitotic activity in the “winter” germinal masses decreased considerably but did not cease completely. Degenerative processes involving germinal cells and young cercarial embryos were enhanced, while the fully formed cercariae, metacercariae and adult flukes in the redial brood cavity were not affected by degeneration. Degeneration products were absorbed and digested by the cells lining the brood cavity, whose ultrastructural characters indicated an intensification of the digestive function. Feeding on their own degenerating germinal elements during the cold season appears to help the rediae to sustain their own life and that of the advanced stages of the hermaphroditic generation contained in them. In addition, the use of internal food resources presumably makes it possible to decrease the parasitic load on the molluscan host organism during the hardships of the cold season, which promotes the survival of the rediae group. Our data suggest that a traditional view that the functional activity of trematode parthenitae stops completely in winter should be reconsidered.